Hi,
Seems to be a problem that I had twice. I believe it is you power supply.
Have you recently added any component in your computer?
- Blu Ray, DVD Burner, Hard Disk, New Video Card, etc...
Do you have enough watts in your Power Supply?
Do you smoke beside your computer or if you open the case, is there any dust in it or around the case?
If yes, it could be the problem of your computer.
Smoke(tar) and dust are the 2 worse enemies for a PC.
The power supply is an important thing in the computer, it's managing the flow of the electricity in the computer. If you have a cheap one, it could also result the problems you have now.
I suggest you to pay minimum 80$ USD. For a Power Supply and be sure to have the good Wattage for it also verify if it's an ATX.
Excellent brands are: Antec, Corsair, OCZ, Thermaltake, Zalman.
If ever you open your case to clean your PC, don't use your home vaccum! You will need Q-tips and alcohol 90%.
At 90%, the alcohol will evaporate really fast and it will not damage your system. If you don't want to clean it by yourself. Technicians will do it for $$.
Here some links to help you to find a good one.
http://www.sohodiffusion.com/components/power-supply.html
http://www.tigerdirect.com/applications/Category/category_tlc.asp?CatId=106&name=Power-Supplies
http://www.newegg.com/Store/Category.aspx?Category=32&name=Power-Supplies
The 2nd possibility could be a virus. Make sure your Anti-Virus is up to dated. If ever you don't have any, well it's about time to have one, but even you try to install an anti-virus on an affected computer, you are not going to fix your problem, because the AV will probably not see the Virus.
I will suggest you to re-install Windows, then do your updates (SP2-SP3 + others) and Install an Anti-Virus. (An excellent one: Antivir. It's free and it's better than Norton and Mcafee!)
Link: http://www.free-av.com/en/download/index.html
So if after the re-installation of Windows your system is still crashing/freezing, I really believe it's your Power Supply.
Hope it will help you!
Good luck