Insert your XP installation disk and restart the computer...when prompted, press "any key" to boot from the disk. When the disk is loaded, select "R" for repair... Select "1" and press enter, then enter your administrator password and press enter(if there is NO existing password just hit enter)...Next at the prompt type: cd \ and press enter
Next, type: copy d:\i386\ntldr and press ENTER
Once the file is copied, type "exit" and reboot the computer, and see if it worked