20GB inside Program Files? My old PC had a 20GB hard drive! Microsoft must have shares in Seagate and Western Digital, for .net to take up 1GB just for what it is.
My entire Windows and Program Files folders have a combined total of 3.42GB according to the properties dialog. 23255 files, 2062 folders (and no games/movies/MP3s at all on the C: drive, which I always install on a second, larger drive). After going with a 2 drive system, I would never go back to having only a single hard drive. With one drive, as soon as that page file kicks in, you'll be lagging like a fly in molasses.