I have been battling a low disk space alert on the C drive on my SBS server for the last couple of days. The problem is that my disk space on the C drive should not be changing. Most of my databases and user files are on a different partition. I eventually found the culprit, sbsmonitoring. For some unknown reason the sqlagent for sbsmonitoring was set to manual and was not running. I guess that sbsmonitoring has an SQL maintenance plan that cleans up and optimizes the database every day. After a couple of days the sbsmonitoring database and log files are much smaller.