Computer freezing problems

I just bought the components for a new computer and put it together on Tuesday. I tried playing Guild Wars 2 on it Wednesday night, but I started having problems, and they have persisted. While playing, the game will freeze and cause my computer to freeze as well. The sound still works though for the most part. I can still hear all the things that are going on and I can hear my friends talking, but it has freezes in it as well.
My build is:
i5 3570k
EVGA GeForce GTX 660 ti
ASRock Z77 Extreme 4
Crucial Ballistics 8gb RAM (2x4)
Samsung Spinpoint 1Tb Hardrive
Cooler Master 500W ATX12V PSU
I have speedfan running to check my GPU's temp, and it is usually between 42C and 48C, so I don't think it is an overheating problem, but I have no idea what could be causing this. The weird thing is that it let me play Guild Wars for about twelve hours yesterday without any problems, but I can't do anything game related on it today without it freezing up. Also, I've tried Alt+Tab and Ctrl+Alt+Del to get out, but nothing works. Does anyone have any suggestions on what it could be or what I could do?
Oh, I updated my graphics card drivers to the new one, but it started crashing, so I switched to the previous driver, and it still did it. I also read someone say that maybe their card wasn't getting enough power, so they underclocked it. I downloaded MSI Afterburner and underclocked the Core and Memory by about 150, but that didn't really help. The computer has froze outside of games once.