I just want to check that when you say all your data files are stored on other disks, that you are including system databases, such as TempDb. These are stored in the SQL Server Program Files folders by default (this also can apply to .BAK files as well). If you haven't done so already, these files can be moved off of C: to clear up some space.

Here's a link to an article describing how to move TempDB:

http://www.sqlservercentral.com/articles/Administration/63568/

Other ways to free up room on C: are to remove the C: drive's pagefile.sys (note: doing so can affect your ability to debug OS issues):

http://support.microsoft.com/kb/307886

Also, if your ReCycler folder is large (you have to show Hidden and Protected Files to see it) that indicates that some user(s) on that box has a large Recylce Bin that has not been emptied; you can try to delete the contents of this folder (note: user will not be able to recover any of the current contents of their Recylce Bin once you do this):

http://support.microsoft.com/kb/229041

Hope this helps.