Vista freezes when idle. How to find a source of the problem?

Vista freezes when idle. How to find a source of the problem?

"I'm using Dell xps 1530 laptop and everything worked fine until last week. Compleatly out of the blue Vista started to freeze. But it doesn't freeze while it's used but only if it's idle. Really nothing special happed last week (or perhaps two weeks ago) only a couple of automatic window updates, so it difficult for me to find the source of the problem.
Vista freezes after computer goes to sleep so I turned that off, screensaver doesn't seem to affect the issue and freezeing still occures. THe freeze itself is strange because mouse cursor is not freezed, and I am allowed to change screens for a couple of seconds but after that (when I realize that the software is no responding) the only solution is to hard reset.
Is there any way to pin the source of the problem? I know that it will be difficult for you to find the cause of freezing but perhaps a copule of guides where to start serching for it will allow me to solve it by myself."