USB problem or keyboard problem? Keyboard might be messed up and causing the letters or phrases to repeat. The keyboard could be screwed. I would try another one and see if the same thing happens if possible. Or find somebody with a USB keyboard and see if you could borrow it. If its a USB bandwidth problem or something with win98 then it should happen with all keyboards. This sounds like more of a defect in the keyboard because it stops when you pound the sh|t out of it.
A wise man once said, "Don't piss on my leg and tell me it's raining."