i have the same problem too. the only answer i could come up is this one: windows is freakin' stupid !!!
after i installed windows, with everything i had 2.21 GB free on c:
then i watched a few movies... restarted... (didn't install anything)... the next day i see i have only 1.82 GB free on c: !!
and no, it's not the swap file cos i set it to 512 MB right after it finished installing. it doesn't grow bigger except if i'm playing high-requirements games.
must KILL BILL !!!!!!