Cleaning a Computer

Yup, thats fine, does it have a straw attachment to direct airflow at all? if so, then use it,
make sure your Case and cpu fans are unplugged or otherwise prevented from moving, as spinning fans with aircans is fun, but generates electricity back to your mobo and potentially fries things,
also be wary of any moisture from the can, test spray first to remove any liquid from the nozzle, small droplets can be in there,
spray from a good distance and use a soft brush or cocktail stick to loosen any stubborn dustbunnies
as long as you are careful you will be fine  
Moto

Air cans vs Dusty PC = AWESOME! You'll feel like a dude in an FPS with a flamethrower!

Just absolutely remember, do NOT tip the can upside down. The gas will actually cause frosting which is moisture which can actually cause shorts and fry things! And as Motopsychojdn said, spinning a fan with the air can cause electricity to be fed back into the motherboard. IT works the same way a generator would; however the voltage may be so low that you'd think it wouldn't fry something. But remember, most of todays modern processors run off of as low as 1v. So a 2v surge could effectively burn the thing up.

One thing I wanted to suggest is if you haven't done it in a while, take it outside! It can become extremely dusty in a low ventilation area and it's just not good to breathe that much dust at once.

Thanks a lot guys! I'm going to clean out my old dust collector tomorrow! XD