6Mb for the C drive? Is this a laptop? By the time you have added perennial favourites like Adobe Reader and all the other crud that infests all of us, (just thrill at those temp files) I would say 20GB gives a bit of elbow room, but not much. After 12 months, C drive is a jumbled mass of... err... things. If you are going to do the reinstall every 6 months, then you are either a masochist, or seriously short of space on your C drive.
One day, I will start a thread to see who has the most unwelcome, uninvited, intrusive, nasty, useless, unpleasant, waste of time items in their start up list.
Honestly, give yourself some space, because however rigorous you are, C drive will fill up like a toddlers nappy.
Conan