I was also wondering if i could get XP on my computer without the CD and without it being already installed on my computer.

> "Windows Vista under one user account and XP on another?"
What do you mean by that?
If you mean a screen where you click the user Joe and it loads XP with Joe's account and you click Harry and it load's Vista with Harry's account then no that isn't possible.
You can have both XP and Vista installed at the same time, have a screen to pick which OS to boot and then select an account in that OS and login with it.

> "XP on my computer without the CD"
No, it is not possible to install Windows XP on a Vista computer without the cd.