Try cleaning the PC with CCleaner. Do the Clean and Registry both. Defrag.
If it still crashes, it could be a graphics driver issue. Uninstall the existing video driver and then run Driver Sweeper from Safe Mode (F8 at bootup). Have it remove any remnants of Nvidia and AMD drivers it finds. Re-boot and install the latest driver for your card/ O.S.
Driver Sweeper: http://phyxion.net/item/driver-sweeper.html
Installing a Q6600 in place of your old processor may require a BIOS update. But if the board recognizes the processor at bootup, probably not. (I believe your board is a Foxconn OEM board. You may be able to search for a Foxconn PT 890-8237A BIOS online if you need one.) When you started Windows after installing the Q6600, did Windows say it was loading the driver for that processor and then ask to re-boot? If so, the O.S. probably is good with the change.