boot from the xp cd. let it get to the screen where you decide what to do(welcome to the setup, i think). it is something like this:
This portion of the Setup program prepares Microsoft
Windows XP to run on your computer:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
Press R to start the console. It will ask which windows install you want to login to. I imagine there is only one, so choose 1:, if you set a password on the admin it will ask that now. once in the console run these commands:
fixmbr
fixboot
I think fixboot will fix it. I am not sure though, so do both. They won't hurt anything. That should fix it. Good luck.