Download very small super grub2 disk
and burn it to CD, boot the CD and select the pre-selected option to detect any OS, shortly after it will show your two Debian booting options, normal and recovery, and the Windows XP option. Select the Debian normal boot option and hit enter and it will boot you into Debian. Open a terminal and run the fdisk command as root or sudo to find which disk your installations are on. If indeed they are on /dev/sda, run command as root or sudo: update-grub
then command: grub-install /dev/sda
to install to the MBR. Reboot and there should be a menu to select either Debian normal, recovery or Windows XP.