MMM-A

If you're worried about stored charge, trying to turn the computer on with it physically disconnected from the wall should bleed off any stored charge.

I agree with Jaron that you really don't need to open it. A nice strong vacuum and an old paintbrush or toothbrush to loosen the dust should do the trick. Don't do the job without a vacuum, or the dust you dislodge will only go somewere else.

Try to vacuum in the reverse of the normal flow of air (whch is from inside of the computer thru the power supply then out) while you dislodge dust with air or a brush. That is, position the vacuum at the 'inside the computer' side of the power supply. Use common sense, though: vacuum in a way that'll keep all the crud from going inside of the supply.

Compressed air is wonderful to use instead of a brush if you have access to it. You can get cans of compressed air at computer supply places - they're sold to blow dust off of keyboards.

If you CAN easily open it - and seems particularly crusty inside - then just use the brush or compressed air to loosen the built-up dust and suck it up with the vacuum.