Try these steps:
1. Test memory with MemTest86 (http://www.memtest86.com/
2. Upgrade to latest video drivers and MB drivers
3. Are you overclocking? Don't.
4. Check your bios to see if your RAM setting is "optimal." If it is, change it to "sync."
5. Clean memory. (To do this, take the ram out and go over the contacts lightly with a pencil eraser; then, use a q-tip with alcohol and go over them with that. Blow it to get rid of cotton fibers but be sure not to spit on it.)
6. Go to windows recovery mode and run chkdsk.
7. Makes sure your video card isn't overheating. To test it, open up your case, take out all the cards surrounding the card, and blow a regular fan in to the case. If the computer doesn't freeze when it's like this, it was probably your vid card overheating.
8. Make sure SP2 is installed on your machine.