char *arg[5] ;
arg[0] = "grep" ; arg[1] = "-n" ; arg[2] = string.c_str() ; arg[3] = fileName.c_str() ;
arg[4] = NULL ;
execvp(arg[0], arg) ;

#include <iostream>
#include <fstream>
using namespace std;

int main()
{

	fstream fin;
	char fileName[21];<< MS-Windows filenames can be 255 characters
	char line[500];
	char string; <<< this is NOT a character array, only ONE character

	cout << "Enter the name of file you wish to open: ";
	cin.getline(fileName, 21);
	if(!fin)
	{
		cout << fileName << " could not be opened. \n";
		return 0;
	}
	int lines = 0;
	cout << "Enter your search string: ";
	cin >> string;<< use getline() here so that the
search string can contain spaces
What is the below for?  useless.
	if(string > 65 && string < 122) << ?????
	{
        fin.seekg(0,ios::beg);
		cout << string << endl;
		lines ++;
	}
Well you now have the filename, so you need to
declare an object of type ifstream, open the file, read each line, search the line for the search string
and finally close the file.  Looks like you still have some work
ahead of you.	  
   
	return 0;
}

bool find_string(ifstream& file, const string& search)
{
  string line = " ";
  while( getline(file, line, '\n') )
  {
     if (line == search)
     {
        return true;
     }
  }
 
  return false;
}

#include <iostream>
#include <string>
int main( int argc, char **argv )
{
  std::string number ;
  std::cout << "What number do you want to factorize: " ;
  std::getline( std::cin, number ) ;
  std::string factors ;
  // so I'm stuck here, I could use some help implementing the general number field   
  // sieve, perferable in a easily paralizable manner so that I can use all my 
  // availible computers to solve the problem...
  // ofcourse if you have a faster algorithm for 100+ digits numbers I would appreciate
  // an implementation of that as well....
  std::cout << number << " factors into " << factors << std::endl ;
  return 0 ;
}