I don't particularly like Windows Vista, but it play beautiful Blue-Ray Movies as well as awesome graphic intensive strategy games.
However, I started noticing a very long lag at startup. In particular, a long time after the loader and logo. Particularly when it is logging into my user.
Further research showed that it had to do with my Video Cards. Not really because I had two video cards, but mostly because of Service Pack 1. Which, ironically, made it harder to install Service Pack 2. Neither of which really solved the issue.
In an HP users forum (from all places) laptop and desktop users had described the EXACT same problem with the black screen lagging.
Disable a setting called TMM.
This is in the Control Panel> Admin Tool> Task Scheduler>Microsoft>Windows>MobilePC>TMM
Right click and set disable.
Reboot and see if it worked.