If you format the 10.04 partition, this might erase GRUB stage 2, but it's a cinch to reinstall. If you don't like to get your hands dirty on the command-line, just boot the 12.04 CD and install the "boot repair" program (https://help.ubuntu.com/community/Boot-Repair).

The first button in the program, "Recommended Repair" should work to install GRUB again. Or you can go into the advanced options and choose a new place for GRUB.