Boot from your Windows XP Home/Pro/MCE CD.
Where is says "Press enter to install windows", press "R" to jump to the recovery console.
Now, i recommend to do the following. If you're administrator account has a password, you'll need to type it in.
Run fixmbr . It'll issue a warning that it could corrupt your harddisk, but usually you'll be right. Follow the prompts and when finished, type fixboot . Fix boot will rewrite windows xp loader code, and then type exit to reboot. Don't boot from CD, and XP should be resurectted from the dead.