When you did the upgrade, you should have let the installer update your GRUB boot information. From your description, I suspect that you chose some other option.
On the other hand, if that's the problem, you should have gotten a "File not found" message.
In any case, boot from the DVD using "linux rescue" at the prompt (or from the rescue disk -- CD 5, I believe) do a chroot /mnt/sysimage to your upgraded installation, and try a grub-install to reinstall GRUB.
Of course, if the rescue fails to find your new installation, you'll need to try other things.
CAUTION: Create an XP boot floppy first so you can boot to your XP system if something goes wrong.