Computer freezes too much, something is wrong

My Vista laptop will run normally, maybe a little slow, until enough things are open or enough time has gone by with something open. Then it will show signs that it is about to freeze, like whatever I'm looking at stops responding (but no error message shows), the taskbar will not come out of autohide, and control-alt-delete does not work. Eventually the screen freezes. No keys work, the mouse does not work (it [and its tail] are frozen with everything else), and even the power button does not work unless I hold it down and do a hard reboot. That is the only way to get out of it.

I've tried running Microsoft Security Essentials. It always gets stuck on the file nlslexicons. Even when I left it overnight, it never got past that file. I tried running it in safe mode, and it got stuck on mssphtb.dll.mui . I've deleted McAffee and other scanners. I tried doing a quick scan with a scanner on the Microsoft site and it found no problem.

I deleted all temporary internet files, cleared the recycling bin, cleaned the disk, and it has made it a little better but has not stopped the problem.

I need help... I cannot use my computer for anything without it completely freezing. I've also left it overnight when it was frozen, and it was the same in the morning. I don't know what I should do.