This is a well know bug with OS 9.0.x. Download and run AutoPurge, which you can get from VersionTracker, or update to OS 9.1 which fixes the problem.
If you are running TechtoolPro, you should also go to that control panel and make sure you have set a limit for the size of the TrashCache. If you have not done so, purge the trashcache, then set a reasonable limit to its size.