"This isn't a known bug, so you'll have to search for it.
First stop is the Event Viewer (in Start / Programs / Administrative Tools). Open Windows Logs, and look for errors in the sub-items. This might point you at some error.
Second, check if your system is fully patched. Start Windows Update and see which updates are proposed. Pay attention also to the optional updates, and especially for device drivers.
Third, to check if this is a Windows problem or some application's fault. Reboot in Safe mode (by pressing F8 at lot during the boot, until you're presented with the boot menu).
If this is still happening in Safe mode, then something is wrong inside Windows and all bets are off. You might need to do some heavy and perilous manipulations to Vista.
But if the problem disappears, then some application is causing this, and you would need to find out which. Since this is probably an application that starts with Vista, best bet is then to download Autoruns and look for superfluous startups (I suggest that you first take backups, make a system restore point, and use Autoruns's option to store the current state so you'll have several means to undo your actions)."