This problem happened after installing the Windows 7 theme.. whenever I play a game (GTA SA, IV, whatever) an error comes up saying something about insufficient space on my drive and then I get a choice of closing the game or not.
I tried going back to the Vista theme but didn't help.. I've tried System Restore but it didn't go far back enough..
(I also added a new value in Regedit 'MinWidth' of 0x56 to make the taskbar programs look like W7's, deleting that didn't help at all either)