First I would recommend you try to find clues about the crash in the Windows event logger: http://kb.iu.edu/data/aivi.html
If you are not lucky there then the following reasons could cause the freezes :
- a virus : check your PC with a complete anti-virus scan and make sure you are clean.
- your hard disk is almost full : if your hard disk is over about 80% full and you are running several programs at once then it might be that your disk has trouble coping.
Try to delete some unused programs or transfer videos/music to your external hard disk.
- hardware issue : in that case I suggest you run a Linux live CD to test if this is a hardware issue or not. Don't be afraid this is very straightforward to check even if you have never touched Linux before. You only need to know how to burn a CD and boot from it.
You will be asked if you want to try Ubuntu at the start. Choose yes and Ubuntu should be up an running shortly. If you encounter no freeze then the issue is due to your Windows installation.
Steps:
1. get the ISO file from Ubuntu: http://www.ubuntu.com/desktop/get-ubuntu/download
2. burn the ISO on a CD (http://www.petri.co.il/how_to_write_iso_files_to_cd.htm)
3. boot from your CD (http://pcsupport.about.com/od/tipstricks/ht/bootcddvd.htm)
cheers.