Any files...you cannot access data files which are not in system file folder (My Docs, My Music, Documents & Settings, etc.)?
I don't have Vista, but I've never heard of anything related to Vista which would preclude an XP system from accessing ordinary data files.
System files/folders are a different matter. Even so, they should be able to be accessed by the following procedures: How to take ownership of a file or folder in Windows XP - http://support.microsoft.com/?kbid=308421
All of this assumes, of course, that malware and hardware problems are not present.
And, no...there are no additional drivers involved in accessing a hard drive which has been attached to a system as a secondary drive, either via USB, by network, or by IDE/SATA motherboard connection. If you were using a 3d-party controller, yes...you would need a driver for that, but it doesn't apply here, based on what you've stated.
You do see the attached drive in My Computer?