It you configure the GRUB bootloader correctly, you will have a menu asking what OS to boot. However, it sounds as though your bios does not like the MBR. If you don't know how Grub functions, I suggest you go to
Read their documentation. It has meanings of all the errors, commands, etc.
When you reboot, is there a menu asking for your OS? Because, if not, you likely have a Stage 2 Grub error, and no amount of reinstalling will fix that. You're best bet is to research the nature of the error and then fiddle with the bios until it boots properly.
I think your problem is with Grub rather than windows. If you get Grub to boot to the "OS Menu", XP should boot. If not, in the boot config file, replace the windows boot script with
Also, if you happen upon a Grub command line GRUB> , you can type those commands in, in that order, to boot windows almost surefire. Unless you're windows is corrupted.