System freezes and reboots improperly

I am running an asus mb, solid state hdd (60 GB) with a 650w power supply with windows 7 (home premium). the system "freezes" after about 5-7 minutes every time (although the cursor/mouse can still be moved, but it won't do anything). if i leave it alone, it will eventually have the screen go black (except the cursor, which can still be seen and moved) and then eventually restarts - although it often cannot boot without my interfering and pointing it to the HDD drive to boot. Any ideas? I have checked CPU temp (40-46 C) and run chkdsk. cant run full virus scan because system freezes before it's done. i have Express Gate and have tried to work with that instead of windows, but it also freezes...HELP!!!