On the comp I built(with friends help!) over the summer, we partitioned the drive, and I couldn't remember which section we left for programs (we have Linux on one section, windows on another, and the third is for any programs {games, et cetera} I install). Anyways, I forgot which drive I had set for installing programs on, so Windows started going "Hey! We don't have enough room left to run anything!" But I un-installed and then re-installed stuff to the proper drive.
That's my random thought