Sulli,
I have sat here and thought about this issue for the past couple of days and unfortunately the only conclusion i can come up with is as fallows.
When you reinstalled a clean version of Win7 and your games didn't work I'm sure that you probably installed all the available updates Windows had available. I would venture to say that in those updates there was an update for your video card that was recently released and that it isn't working to well. This would most likely explain why it worked for you one day and then all of a sudden stopped working. I would try a fresh install and I would not allow windows to do automatic updates, I would do them manually so it don't install drivers that don't work.
Sorry I couldn't come up with a better answer for you, but after all it is a beta and will be prone to problems.