Ok, you need to boot from the XP Pro CD, and when it gets to the part that asks is you want to install or what, choose R (Recovery Console). It'll then boot into a black screen with white text, ask what partition you want to boot to (if you only have one windows install it'll be number 1, then
), and ask for the admin password.
Once you are at the recovery console prompt, type fixboot, and it'll ask if you really wanna do this?, say Y for yes, then once it's finished, type exit to reboot.
If that doesn't fix it, we need to look at restoring the MBR, but that's more problematic