I'm not sure if there is a minimum that it takes for the installer to run, but you can definitely fit it all in under that much. After SP2, I'm still below 800, so if you remove parts that you don't need (make sysoc.inf visible, remove the word "HIDE", then uninstall some additional stuff), you can definitely get it to fit on that drive. However, the page file would put you over and since every users Documents and Settings folder is on that drive, you won't be able to do too much with it.