If you are running out of space on the "C" drive on a system, one way to avoid the hassles of rebuilding the system just to gain space is to use the junction point feature of Windows. This will allow you to move entire folders from the C drive to another drive, thus freeing physical space.
There is a Microsoft KB article on how to create and manipulate NTFS junction points and another article with the best practices for Sysvol maintenance.
Windows does not come with a tool for creating junctions. The Win2K Resource kit includes the linkd program for creating junctions, but if you don't want to purchase that there is a free Sysinternals tool called Junction that can be used to create junction points in Windows.