Vista will still see the drive you just wont be able to go into account information for XP. The file will still be there as long as you don't save it in My Documents. Create a separate folder that you can share info between the 2 OSs. C: can be your global as long as you install an OS on it. I have XP running on Drive I and Vista on C: The reason its on C: is because i was going to use that partition anyway. You can use any drive for OS install.

If you dont have XP CD get one they are much cheaper now. Just make sure you also dont have the upgrade version of vista. You will need full vista to make the dual boot work.

8gb XP
220gb Vista

You will have to look into install order of which OS to install first. Reason being is that you might have to take different steps in each.