Did you check your power management settings? The Turn off monitor can be ok, but if your hard drive is set to turn off after a period of time, this can often cause lockups like you describe. Same with System standby, and hibernation (if you have that enabled). Even your screensaver trying to come on (esp with third-party screensavers) can cause these types of lockups after a delayed period of inactivity.
Beyond that, I can't think of too much that is easy to check to help you figure out why your computer might be doing this, beyond some sort of driver issue.
Also try running a Chkdsk on your hard drive, and afterwords try defragging. You DO have plenty of free hard drive space left?
Could also try running a memory test on your computer. I use Memtest86+ found here:
http://www.memtest.org/
You will have to download it (version 1.70), burn it to a CD, then boot your computer to that CD and let it run, preferably overnight. Along the same lines you could try some good stress tests in Windows (like Orthos or SuperPI) and see if your system locks up during those.