What's the "sign-in page"?
Although we don't recommend using such cleaners, they are unlikely to cause the system to freeze while running. So I think it's unrelated. It could have happened just as well while you were using MS Word to edit a document.
Can you boot into Safe Mode with command prompt?
Can you boot from your Windows Vista DVD and see if any of the options of Startup Repair work? That's described in http://www.bleepingcomputer.com/tutorials/repair-windows-with-windows-startup-repair/