After April 13, 2010, Microsoft will no longer issue security updates for Vista RTM (release to manufacturing). Vista users are encouraged to upgrade to Vista Service Pack 1 or Service Pack 2 if they have not already.
I'd personally recommend SP2.
After July 13, 2010, Microsoft will no longer support Windows 2000 at all, and will no longer issue security updates for Windows XP SP2. If you are still running Windows XP SP2 upgrade to SP3, or Windows 7.