Putting aside typical Windows problems (just screwing up - very common), from my previous expierence there could be two causes, Bad RAM Memory or CPU overheating. There could be more causes however.
First thing I would do would be to check the RAM. There is a good program here that will check the RAM and it boots from a CD. Report back with the results.
Another thing would be to check how hot your CPU is running. To do this I use a program called SpeedFan that can be found here. However, certain people have had mixed success with this program so I'm sure that sammorris will reccomed another one when he looks at this post. Report back with some temperatures and what CPU you have.
This can also occur with a power supply that's going bad. Spikes and surges in power can cause your PC to lock up. In reality, pretty much anything can cause this, including hard drives and motherboards, but some things are more common than others (RAM, Overheating, PSU).
So far, all of the tips involve hardware. However, it's possible that your hardware is fine and that there's something at the software level that's causing this problem. A good test for this is to load a bootable CD, such as Knoppix (link here), and let it run for several hours. If your computer usually locks up frequently, and it no longer does, then it could be a hard drive or software issue. If you computer continues to lock up with a bootable CD, then it's definitely a hardware issue.
Give some specs on your system. Is it custome or retail. I would test your ram google memtest its a free down load to test your ram and run speedfan to see what temps you are running newer intel the tjunction is 100 so you wil need to put 15c offset in.
