A lot of people are having various problems with Vista and companies are still buying PCs with XP. For example Dell will supply a new PC with XP loaded and a VISTA CD to allow you to upgrade at the right time. Have a read around in various forums and you will see plenty of comments.
SP2 is Service Pack 2 which will contain all the bug fixes and upgrades ... it will appear sometime in the future.
Obviously, you may find it works fine for you and if you are not confident of doing such a change, then do not risk it.
M H C
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