1st question: to know whether the issue is something maxing out the CPU, when you say you don't see it in Task Manager, does Task Manager continue to actively report changes in activity or does it also freeze (but could be freezing before it is able to report the problem)?
Do you have more than one antivirus program running in the background? That will do it.
If it is neither of those, it is likely either corrupted software that is sending your computer on a wild goose chase or a hardware problem (probably heat-related). Is the computer fine when you first turn it on and the problems start say 10 minutes to half an hour later?
Run sfc /scannow from the Start menu Run window, then reboot. If the problem is a corrupted windows file, that will find and correct it. See if the problem disappears. If not:
Try booting in safe mode and just leave the computer on for an hour or two. Occasionally try to do any activity just to see if the system is frozen. If it freezes, it is either Windows corruption or a hardware problem. If you have run sfc, that leaves hardware.
If it doesn't freeze, that rules out hardware and the core of the operating system and points to software--either something non-critical from Microsoft that is automatically loaded at bootup or third party software that is normally running on your machine. Repeat the test with a clean boot:
http://support.microsoft.com/kb/929135 This loads pretty much everything from Microsoft that is normally loaded, but no third party stuff. Leave the computer on for an hour or two, occasionally trying activity to see if it is frozen.
If the system freezes, the culprit is one of the Microsoft programs or services that loads in clean boot but not in safe mode. If it doesn't freeze, the problem is one of the non-Microsoft programs that loads in a normal boot.
