Perhaps you have a run-away query, or a poorly written report running against a database on the server? You can get some info in Enterprise Manager -> <server> -> Management -> Current Activity -> Process Info. Not the best resource, but I think it's pretty much the best built-in way. We had the same issue when someone wrote a report that had a REALLY ugly join. Even caused our tempdb to fill up.

Does the application you have using the database have a means for checking on it's processes and queries? Some do, such as Crystal Enterprise.