i have a compaq pc and had random problem after randon problem after doing a vista upgrade, the hp site seem quite uninterested with vista problems i noticed.
it ran much smoother after a clean install, it was much better, but i have gone back to xp as vista just seemed to hold a different headache every week, i shall possibly put it on a second machine after the service pack is out.
i think with vista it depends on which software you use and which hardware you have. it's a lot prettier than xp but with the compatiblity issues was far to limiting at this present time for myself and after 3 months non of my regular programs seem any nearer or interested in improving compatibilty at their end.
try a clean install, and you will need to download a fix from hp that stops the pc locking up when it comes out of sleep (or just change the power buttons to shut down)