just a quick caveat about warranty...if its a recent laptop and still under warranty, many manufacturers will view you changing the OS as a violation of warranty. ie they want you to take it into a service centre to get done, so if something goes wrong afterwards, you are still supported.
As Rhettski, said, check HP site for the list of supported OS and their drivers.
I understand people wanting to have a dual boot to XP for compatibility issues, but a full roll back is cutting you off from running apps that are/will be vista specific.. This is even more a concern if HP never actually issued you with an original OS disc
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