Here are a few things I would try first.
Do you have a System Restore point before the problem started?
Look in Event Viewer for any errors at the time of the freeze, for a clue about whats causing the freeze.
Boot into safe mode with networking. Start tapping the F8 key while the computer is starting up. Run it for long enough to see if it will freeze.
If things are OK in safe mode, in the search box enter msconfig.exe and click on it. In the Startup tab disable everything except your antivirus app. Next in the Services tab click on Hide Microsoft Services and disable the remaining services. Restart and wait for the PC to freeze. If it is still running OK, go back to msconfig.exe and enable about half the the entries you disabled earlier. Reboot and see what happens. The idea is to find the startup program or service that is causing the problem by adding back the entries until you can find the offender.
Finding the problem will take some and a lot of reboots.