there’s a way to do this if you only need into your XP partition (it will allow read/write access to 7/vista partitions, but not boot them until you redo their bootloaders)
requirements:
winXP CD
take out all drives but XP drive, just to reduce confusion
boot to the winXP cd, and go to the recovery console.
2 commands you need to run, once you log in (should hopefully be able to log into the XP system on your drive, if not, you could be in trouble):
1 – `fixmbr`
2 – `fixboot`
should have XP booting again. I’ve done this before several times when purging Vista from my hard drive. I think this is how it works 