Hold the power button in for 20 seconds till the machine turns off. Boot the screen from scratch and you should have your normal login window..
Unfortunately, I'm not at a Vista machine to check for other alternatives.
Hope this helps.
I'm new to this forum but Ihave found something annoying to say the least in Vista(Unless im missing the obvious)
In XP if the screen is locked (On a domain) you can use <cntrl><alt>+<del> and force someone off if you are domain admin etc.
I cannot see how to acheive the same in Vista business. Once someone is logged in and locks the screen using <cntrl><alt>+<del>+<Enter>
The options just is not there to switch users.