The most common cause of computer freezes is a corrupt registry.
Most likely your registry is bloated and overstuffed with an assortment of useless pieces of data. The operating system while running your computer has to pick its way through all this excess material. There will be bits of programs which were not completely uninstalled, parts of programs which were never completely installed, fragments of miscellaneous data, useless drivers, and a multitude of other bits and pieces.
The answer is to clean your registry and remove all the material which is not needed.
http://fix-computer-freezing.com/Clean_Up_Center.htm
Get the best one, and completely clean your windows registry with a few clicks of the mouse.
Often there is also spyware which is using your computer for nefarious purposes and slowing its operation.
The answer is to scan your pc with a best anti-spyware to clean spyware/trojan/virus.
http://fix-computer-freezing.com