Vista could be slightly picky about the way that it handles file permissions, for instance if on your XP system you had your main account called Todd and the second account on XP called Lisa, but your Vista login is called Myself then sometimes it might be a little strange about having you logged in to Vista as "Myself" but then going to look at the "My Documents", "My Pictures", etc. files under Todd/Lisa on the old XP hard disc.
If that happens and it doesn't show you files you "know" should be there then you can just "Take Ownership" of the XP drive / folders on it so Vista will realize you "own" them and it is all really stuff belonging to the same user since usually it doesn't give you full access to someone else's files.
This has nothing really to do with it being a 2nd drive or from XP... Vista would do that even between two user logins on the same drive...and XP would do the same more or less if you had had multiple logins on the same PC... "Desktop", "My Documents", "My Pictures", "My Music", browser bookmarks / history and stuff are all sort of private to a given "username" on the system so sometimes you have to be a little explicit about getting access to them if your user name isn't the same as the one you're trying to access.
It is kind of confusing, I know, sorry. Don't worry about this too much just be aware of it as an explanation if you are missing stuff.
To save your browser bookmarks or emails saved to the local PC via a POP/IMAP mail program like Outlook or other kinds of "personal" things like that it is usually best to "export" them from your user account while you're still running the old OS (if convenient) and while you're logged into the account that owns that stuff, and save the exported stuff to some easy to find place you'll know to look when you change OS / login / whatever.