Use Compressed Air
Everybody needs to clean their PCs sometimes. They get layered with dust and grime just like anything else. Compressed air in a can is one of the most useful tools you can have for performing this necessary task. By using the included straw, you can finely direct the air into places where it is most needed.
Clean your keyboard by turning it sideways (side down) over a trash can or some other place where dust can fall, and then blow it out with the compressed air. Ensure that you get in between the keys because a lot of stuff falls in between the keys and can even hamper complete contact of the keys (hence unworking keys).
You also want to remove your case cover and blow out the fan blades and other surfaces. Make sure you don't blow the air right onto your PC's innards up close. Give a little bit of distance between the computer and the end of the straw.