The desktop's fan is quite fragile. If you blow into it, it would start spinning. Thus, the best way is to use a pen (or anything similar) to hold it in its position and them blow away the dust. You can alsoconsider the following cleaning kits:
Use your compressed air can to blow out any loose dust particles. You can also use a cotton swab to clean the fan by rubbing the swab against the blades to stick the dust particles. It's a little tedious, but it makes for a nice, clean fan in the end. The bottom of your case will undoubtedly have dust buildup.