Question: I burned a CD on XP using the built-in burning software and I can read the CD on that machine, but when I insert it into my Vista machine, I can't read the files. It shows the correct volume label, and the correct 'free space', but I can't access the actual files.

Am I missing something obvious?

(Both systems are fully up-to-date)

Answer: 
If you didn't finalize the disk then it is only readable in the drive that created it. This will make it look like it's an XP to Vista problem if these are the only two computers that you've tried it on.
Try another drive - it's possible (although I would have thought rare these days) for a disc to be written outside the tolerance of a drive. I would expect this to be a drive/disc issue rather than an OS issue.