In Vista, you have to be careful with UAC (User Access Control). You can only safely disable it immediately after an OS installation, before you do any updates.
If you disable it after the computer has some software and updates installed, it could break things all over the place. The manifestations are really quite bizarre and inconsistent.
Also note, with Vista, you MUST disable any power management or you could suffer system lockups as that was borked in Vista. So badly borked Microsoft decided the best way to fix it was to get rid of Vista and ship a new OS. Well, at least that one of the many reasons Vista had to die.