Well, I cleaned out my heatsinks this morning. A friend of mine suggested that I re-seat the video card, so I did that as well.
A piece of paper or cardboard or something seemed to be wedged where the video card met the motherboard. I'm not sure how it got in there, but somehow it did and was causing my video card's internal fan to not work. The card would get hot (due to the fan not spinning), and I would crash to a blue screen.
I removed the paper, the fan began running again, and I just got done from a 7-hour TF2 run to test it out.

Seems to work fine, so I'll close this topic unless it pops up again. Thanks for your help, guys!