OS is always going to default to using the C: drive for updates/downloads/program installs etc. You can install programs to your D:, but some things will always end up on the C: drive such as Windows updates.
Assuming your user folders are currently located on the C: you could move them over to the D: drive which would likely free a good deal of space on C:. You could also run a program such as, http://download.cnet.com/CCleaner/3000-2144_4-10547048.html
which scans and looks for temporary files that can be deleted and should free some space. There are lots of forum posts about the proper way to move your user data to a partition other then the C: drive, just search for it. Do not just drag them over to the other drive.