So I just installed Windows 7 on my 27" i5 iMac, and it was working fine until just now. It froze completely (I could move the mouse around) for a minute, and then it unfroze. I kept using it, and then it froze again, but this time I had to restart by holding down the power button. It just happened again, and I had to restart again. The only thing I have running is Chrome, and this is a fresh install. What's going on?