I'm going to assume that you simply installed GRUB to the MBR so grab a boot disk and run an fdisk /mbr. That should do the trick for ya.
NO!!! Do not do an fdisk /mbr from a floppy. It wipes ntldr as well as grub (whichever is installed to the mbr). Boot from an XP CD and go into the recovery console. Then, type fixboot and then fixmbr.

Cheers,
cgkanchi