I think this should be in the FAQ (please post follow up questions there).

This is a keyboard hardware problem and not a software or driver problem. I will not go into the details, but your keyboard keys are not individually mapped, they use a matrix of wires with each key acting as a switch. If you press the right (wrong) combination of keys you can ‘block’ other keys from responding.

To see this in action, open up notepad, press and hold the 'a' and 'w' keys, and then tap the 's' key. On my keyboard the input stops when the ‘s’ key is pressed.

To make things worse, different keyboards can act differently.