OK if your vents are blocked with dust the chances are your components inside will have a nice layer of insulation too.
Feel free to take your case off (screws at the back) and hoover out the insides - the more you can get the better.
Dont use a brush implement no matter how tempting (unless its an anti-static brush) as typical implements for a hoover are good for static.
If you can see clogs of dust when the case is open hold on to the metal case (whilst the mains cable is still plugged in so it is earthed) and dislodge with your fingers so it can be hoovered up (holding on to the metal frame is important to stop static).
You may want to nip down pc-world/staples etc for some spray duster. Its an aerosol in a can that will emit a high pressure jet of air to dislodge the dust (usually directed through a straw). That should dislodge everything and make your computer last another year or so !