I suggest you take the PC to a local tech for this, from the sound of your post you don't have much computer experience and the fix to this can easily make the PC not bootable.
The quick answer is you need to edit the boot.ini file, and either remove references to the second OS or set one as the primary and turn the time-out to 0. If you want to totally remove it from the drive, the best thing to do is to back up your files, and start a new XP setup after formatting the drive.