Make sure that you have a Windows rescue disk with the recover console so you can do the
fixmbr thing to restore the XP loader.
Once you do that then you'll be free to format the Linux partition without fear. You should be able to do it from Windows XP, but if you run into problems then boot up with a Linux disk (like tomsrtbt rescue floppy or knoppix) and run the dd command to wipe the drive, or use the harddrive manufacturer's utility to wipe it for you.
the command would be something like "dd if=/dev/zero of=/dev/hdb" as long as you know which /dev/hdb device your harddrive corrisponds to (hda = primary master, hdb =primary slave, hdc = secondary master, hdc = secondary slave).
That will zereo out your harddrive. Then it would be easy to format it with any tool with no risk. (don't accidently wipe out your WinXP installation!!)