XP hard drive used as slave on vista

[Celegorm]

Sort of hijacking this, but say both of his HDDs where SATA. How would he go about fixing the problem?

Computers boot off of the lowest number SATA port so as long as the Vista drive is in the lower number port, it'll be fine.

You'll probably need to take ownership of the files/folders on the XP drive to be able to access the data

Only if the XP had a password on it that he didn't remove. Thankfully, vista makes this a lot easier than XP as you don't need to be in safe mode.