You will need to delete older files in the Completed Queue list. If you have the latest version installed the files will delete after 24-25 hours. If you do not have the latest version installed you should have this installed because there was a bug in the previous version that prevented files from automatically deleting. If files are deleting properly after 24 hours and you still have this I would suggest adding physical disk space. We are running with 250 gig. This is larger than what was recommneded but even with the files deleting automatically we needed more space because of the volume of files we are running.
You will want to use the Details Tab in Task Manager to see system background tasks that may be using memory. Windows will aggressively use available memory for indexing and background tasks when user tasks leave it available. But, the moment a user task needs that memory, it will be freed up. So unless you are seeing a problem, I would not worry about 50% usage.
thanking you :)