The user is asked to input a number in either Hex, Octal or Decimal, which is then converted to Hex, Octal & Decimal.
The user can exit the program from the MAIN MENU or by the Escape key after each convertion.
Compiler : Borland Turbo C++ v4.5.

Comments
Nice program, too bad you used such an old obsolete compiler.
//****************************************************************************//
//                                                                            //
// Program Name : ConvDec.cpp                                                 //
//                                                                            //
//   Programmer : K.Angel                                                     //
//              : Novice C++                                                  //
//                                                                            //
//  Description : The user is asked to input a number in either Hex, Octal or //
//                Decimal, which is then converted to Hex, Octal & Decimal.   //
//                The user can exit the program from the MAIN MENU or by      //
//                the Escape key after each convertion.                       //
//                                                                            //
//     Compiler : Borland Turbo C++ v4.5.                                     //
//         Date : 16/03/2006                                                  //
//                                                                            //
//****************************************************************************//

#include <iostream.h>
#include <conio.h>             // for clrscr() & getche() functions...
#include <stdlib.h>            // for exit() function...

const int ESC = 27;           // Used for escape sequince

int main()
{
	int key = 0;
	int choise = 0;
	long number;

	while(key != ESC)
	{
		cout << "Choose Your Number Base\n";
		cout << "\n\t1. Hexadecimal.";
		cout << "\n\t2. Octal.";
		cout << "\n\t3. Decimal.";
		cout << "\n\t4. Exit.\n\t       ";
		cin  >> choise;

		switch(choise)
		{
			case 1: clrscr();
						 cout << "Enter a Hex number: ";
						 cin  >> hex >> number;
						 cout << "\n   value in octal = "
						 << oct << number << endl;
						 cout << " value in decimal = "
						 << dec << number << endl;
						 break;
			case 2: clrscr();
						 cout << "Enter a Octal number: ";
						 cin  >> oct >> number;
						 cout << "\n       value in hex = "
						 << hex << number << endl;
						 cout << "   value in decimal = "
						 << dec << number << endl;
						 break;
			case 3: clrscr();
						 cout << "Enter a dec number: ";
						 cin  >> dec >> number;
						 cout << "\n   value in octal = "
						 << oct << number << endl;
						 cout << "     value in hex = "
						 << hex << number << endl;
						 break;
			case 4:  clrscr();
						 cout << "Program terminated by user...";
						 exit(0);
						 break;                 //Unreachable code....  
			default : clrscr();
						 cout << "ERROR ~ Invalid selection\n\n";
						 break;
		}
		cout << "Press any key to continue or 'Esc' to exit";
		key = getche();
		clrscr();
	}
	cout << "Program terminated by user...";
	return 0;
}

Move Clrscr() to between cin and switch and drop all the other clrscr() at case 1: thru case 4: and default:.
You are always Clrscr() anyway so why use the case at all for this?
This also saves some typing?

>>8000fe80 why this code is not working for this hexa decimal address
Data overflow.

On line 73 add cin.clear() to clear out the errors.

here's d code i tried....it worked for all conversions

//final conversion-only for decimal input
#include<iostream.h>
#include<conio.h>
#include<string.h>
#include<math.h>
#include<process.h>
main()
{

//too long....the hex case ruins it all and too many switch cases
      
      char a[30],ch;
      int len,i,j,pos,whole=0,k=0,n=0,r[30],lenr=0,lenq=0,q[30],h=0,base,base1;
      float frac=0;
      cout<<"Enter number ";
      gets(a);
      cout<<"\nEnter choice-\n1. Decimal (d/D)\n2. Octal (o/O)\n3. Hexadecimal (h/H)\n4. Binary (b/B) ";
      cin>>ch;
      len=strlen(a);      
                              switch(a[0])//for converting the no. inputted into frac and whole part
                              {
                                        case 'O':
                                        case 'o':base1=8;break;
                                        case 'B':
                                        case 'b':base1=2;break;
                                        case 'H':
                                        case 'h':base1=16;break;
                                        case 'd':
                                        case 'D':base1=10;break;
                              }
                              switch(ch)//for final conversion according to choice inputted
                              {
                                        case 'O':
                                        case 'o':base=8;break;
                                        case 'B':
                                        case 'b':base=2;break;
                                        case 'H':
                                        case 'h':base=16;break;
                              }
      for(i=0;i<len;i++)//code for separating the fractional and whole part
      {
      if(a[i]=='.')
      pos=i;
      }
      for(i=pos-1,j=0;i>=1,j<pos-1;i--,j++)
      {
                        if(a[0]=='h'||a[0]=='H')
                        {
                            switch(a[i])
                            {
                                        case 'A':whole+=10*(pow(base1,j));break;
                                        case 'B':whole+=11*(pow(base1,j));break;
                                        case 'C':whole+=12*(pow(base1,j));break;
                                        case 'D':whole+=13*(pow(base1,j));break;
                                        case 'E':whole+=14*(pow(base1,j));break;
                                        case 'F':whole+=15*(pow(base1,j));break;
                                        default:whole+=a[i]*(pow(base1,j));
                            }  
                        }
                        else
                        {                
                        a[i]=a[i]-48;
                        whole+=a[i]*(pow(base1,j));
                        }
      }
      for(i=pos+1,j=1;i<len,j<=len-pos-1;i++,j++)
      {
                        if(a[0]=='h'||a[0]=='H')
                        {
                             switch(a[i])
                             {
                                         case 'A':frac+=10*(pow(base1,-j));break;
                                         case 'B':frac+=11*(pow(base1,-j));break;
                                         case 'C':frac+=12*(pow(base1,-j));break;
                                         case 'D':frac+=13*(pow(base1,-j));break;
                                         case 'E':frac+=14*(pow(base1,-j));break;
                                         case 'F':frac+=15*(pow(base1,-j));break;
                                         default:frac+=a[i]*(pow(base1,-j));
                             }
                        }
                        else
                        {                                            
                        a[i]=a[i]-48;
                        frac+=a[i]*(pow(base1,-j));
                        }
      }
      if(ch=='d'||ch=='D')
      {
                          cout<<"\nConverted Decimal no. is "<<frac+whole;
                          getch();
                          exit(0);
      }
      else
      {
          
                              while(whole>=base)//coverting whole part
                              {
                                            r[n]=whole%base;
                                            whole=whole/base;
                                            n++;
                                            lenr++;
                              }
                              while(h<4)//coverting fractional part
                              {
                                         frac=frac*base;
                                         q[h]=frac/1;
                                         frac=frac-q[h];
                                         h++;
                                         lenq++;
                              }                   
      cout<<"\nCoverted no.is ";
      cout<<whole;
      for(n=lenr-1;n>=0;n--)
      {
                            if(ch=='h'||ch=='H')
                            {
                                            switch(r[n])
                                            {
                                                        case 10:cout<<"A";break;
                                                        case 11:cout<<"B";break;
                                                        case 12:cout<<"C";break;
                                                        case 13:cout<<"D";break;
                                                        case 14:cout<<"E";break;
                                                        case 15:cout<<"F";break;
                                                        default:cout<<r[n];
                                            }
                            }
                            else
                            cout<<r[n];
      }
      cout<<".";
      for(h=0;h<lenq;h++)
      {
                         if(ch=='h'||ch=='H')
                         {
                                            switch(r[n])
                                            {
                                                        case 10:cout<<"A";break;
                                                        case 11:cout<<"B";break;
                                                        case 12:cout<<"C";break;
                                                        case 13:cout<<"D";break;
                                                        case 14:cout<<"E";break;
                                                        case 15:cout<<"F";break;
                                                        default:cout<<r[n];
                                            }
                         }
                         else
                         cout<<r[n];
      }
      }
getch();
}

How can i convert hexadecimal to binary? Or kindly give me a c++ codes to convert it. Reply please. ASAP.