There may be several reasons why your computer may be freezing. First question is when was the last time you cleaned your registry. Having a corrupted registry can cause a whole host of errors and problems. Freezing up is one of them. If you have a lot of unnecessary registry keys that are hogging up your computer resources. Your computer is bound to choke. Scan your registry with a good registry cleaner. If you pull up more than 20 errors or duplication, clean your registry.
2. Depending on your RAM specs and your computer usage. This could also be a the issue. Having an insufficient amount of free space does not allow your RAM to function properly. If you are using a lot of RAM by net surfing or other activities without enough empty space your will get freezes. You don't need to buy new RAM but you should increase your free space by removing unnecessary data and perhaps increasing your virtual memory. The below link will show you how.
3. Spyware could be the culprit, so do a scan and see if your computer is infected.
4. Another area is a hardware drivers. This could be the problem if new hardware has been added to the system. So, the drivers should still be up to date. So try the first two suggestion and see if you can get your computer to stop freezing.
Check your drivers.
5. Lastly, check the dust buildup on the fan to make sure you system is cooling properly