I agree with Conan totally on this!
C: has to cope with all the program files, system files and user settings (email, My this and My that) as well as all the browser cache and other temp stuff.
It is often possible to push some of this onto another drive (e.g. on installing a new application), but the default is always to C:
BTW - if you are getting short of space on C: it is often helpful to move the swap/page file to a larger drive. Also use compression on C: - e.g. program files.
But anything less than 10GB is futile, less than 20GB a pain!
I have an older PC running XP with a 10GB disk as C: and a 40GB as D: and it is barely coping, with C: about 80% full - and that's with all my tricks employed!
SD