Compressed air mate, its kinda expensive though. A can of air is a few quid and doesn't last very long.
Keep the can upright and use short controlled bursts. Oh and clean heatsinks first. I tend to blow away surface dust, then do the heatsinks, and have to do everything else again cause of the dust from the heatsinks.
Get a can of compressed air and carefully blow the dust off of the components and out the of the fans etc.
Make sure you hold the can at a distance from the PC. Just so that you don't damage anything.
Might be worth taking it outside to clean too.
Case filters are removable so I just remove these rinse them in water you can you a fine brush to clean away dust in the computer along with compressed air