Funny, I just did this last night. My W,S,A,D and Space keys were not responding well. Guess what I have been up to?
Anyways, I have a Microsoft 3000 combo. I used a Phillips screwdriver to take out all the screws on the bottom of the board. Two were hidden under labels.
Then I carefully pried it open. I never got it fully open because there were some sort of latches at the top. I had enough access to get to the guts.
The technology hasn't really changed since I used to clean my Atari 2600 joysticks.
The keyboard has a rubber-like assembly with circuit lines running below each key. There are 2-3 lines that a separated by a few millimeters. Between the circuit impressions and the actual keyboard is another rubber-like membrane that has a cup depression in it. At the bottom of the depression is a small, circular disc made of graphic or similar material. When you press on the button, the cup is depressed and the disk comes in contact with the circuit below it., registering a keypress.
Once you get access to these components, just use some rubbing alcohol and a qtip to clean both the circuit surface and the disk inside each cup. Let it air dry and snap it back together. Don't forget to test before you screw everything down.
Worked like a champ for me last night.