is it the vista transformation pack? if so, I've had problems with that in the past as well. After my last reformat I decided not to put it on there and decided that the trouble it caused, wasn't justifiable. The look was good, but it screwed my PC up. you might want to try deleting the registry entries that it left behind using CCleaner which you can get for free at www.ccleaner.com
Hope that helps!
Desktop: Dell Dimension E510, 2.5GB RAM, Intel P4 3.0GHZ, 250GB HDD, Windows Vista Premium. Might not be the best, but the old girl is still running good.