FILE*myfile;
 myfile=fopen("D:\\Profiles\\DqGw47\\Desktop\\hello.txt","a");
 fprintf(myfile,"%s\n",textbox1->text);
 fclose(myfile);

int main(int argc, char *argv[])
{
  string text = "text";
  string str;
  vector<string> fileContent;
 
  ifstream infile("hello.txt");
  //infile.getline(str);
  while ( getline(infile, str) )
  {
    cout<<str<<endl;
    if ( str.find("Set wallpaper=") != string::npos )
    {
      str = "Set wallpaper=\"";
      str += text;
      str += "\"";
    }
    fileContent.push_back(str);    
  }
  infile.close();
 
  ofstream outfile("hello.txt");
  for (int i=0; i<fileContent.size(); ++i)
  {
    outfile<<fileContent[i]<<endl;
  }
  outfile.close();   
 
  system("PAUSE");
  return EXIT_SUCCESS;
}

open original file
open new output temp file
 
for each row in input file
   call fgets() to read the line
   check for "Set wallpaper"
   if check is true then write new text to output file
   otherwise if check is false then just write the entire row to output file
end of while loop