As per a previous post I have explained one of the most difficult things around maintaining an SBS server, The disk space that is never ending!
Here is a quick list of things you can use to regain control:
1. Shrink Sharepoint Logs (Fix as per my previous post)
2. Delete all temporary IIS files
3. Search for other large SBS log files in the Windows Small Business Server folder (The pop3 connector & Console logs in particular!)
4. Configure update list for the WSUS server and remove any updates you do not need to download, once completed run the WSUS cleanup wizard to remove old update files.
5. I would download and run WinDirStat from http://www.windirstat.info, You can use this free tool to analyze the drive/partition to see where the space is being used up.
6. You can also use the wizards from the sever console to shift exchange, WSUS, Sharepoint etc to another drive/partition if C is becoming to full.