First, pull the battery while it's running to force a soft reset. That will free up all the possible internal memory. You generally need 10+ MB for your BB to function smoothly and correctly. When it gets to that point you can see sluggish performance, deleted messages and calendar appointments, etc. Having 25 MB of free internal memory available is plenty.
Restart the computer you're using to run Desktop Manager as well.
Make sure all your pictures, videos, music, and personal ringtones are stored on the media card.