In this Instructable I'll show you how to properly dissassemble your ( or somebody else's :P ) Xbox 360 PSU.
If all you want to do is get rid of the dust in it, I would simply recommend blowing compressed air through the vent holes.
There could be issues with heat due to little, or bad thermal paste, this will show you how to get to it.
The tools you'll need are a No.2 Phillips, and perhaps a No.1.
Something narrow with a sharp point, preferably strong enough to put a fair amount of force into. I used a hunting knife.
A small screwdriver, or toothpick for applying new thermal paste, or reusing thermal paste.
Step 1: The feet
The first step in dissasembling you power brick is to remove the rubber feet on the bottom. These will pop out fairly easily, and can probably be taken out with your fingers.
I don't think I 'Need' to say this, but make sure the power brick is Unplugged before servicing it.
Once the rubber feet are removed you will see a small plastic insert where each foot came out of. This insert can be pried out by pushing a blade between its edge and the power supply, and prying it up.
I ruined my first one by trying to use the good ol' dental pick, but I found that this hunting knife worked far better, and kept them in a reusable state.