Take a swab or the end of a paper clip and scrape any accumulated grime from each of the guide wheels in the cavity. Rub the mouse ball with a cloth to remove any oil or grime. Clean the keyboard. Blow out dust from between keys with a shot of compressed air. Clean the monitor.
Monitors are literally dust magnets.
Wipe dust from the screen with a damp, soft rag. If you need to use a cleaning solution to remove stuck-on dirt, make sure your monitor has no special coatings that may be damaged by cleaning solvents. Also, remember to remove dust from any vents or openings. A PC janitor's best friend.