64 Freezes And System Not Shut Down Properly

It keeps freezing and has to be re-started using restart switch. There does not seem to be a pattern to the freezing that I can spot. It happens at least once a day, sometimes 4-5 times and the longest without was nearly 2 days (when I thought the problem had gone!). It can happen when browsing using firefox or IE. It can happen when using documents or playing PC games (A Vampyre Story and The Path) Initially after re-boot the system had to be repaired using the Vista CD but now it simply boots to option for safe mode or start windows normally. The perfromance monitor simply shows system not shut down properly,Have removed u-torrent, and peripherals except broadband connection (BTHomeHub 2) and wireless keyboard and mouse but to no avail.

I use McAfee for security and since most recent upgrade have to manually start windows defender using command prompt. BTW by freezing I mean that everything suddenly stops responding and nothing works including any key combos or mouse. Therefore can't access task manager.