Sorry, I was at work, so I couldn't reply.
I'm using usb and windows xp.
There have been threads on here in the past with problems with Windows XP and USB just like the previous poster said. I've got Windows XP and when using the USB keyboard I experience issues from time to time.
For example, may be playing pacman and all of a sudden the keyboard doesn't respond correctly. It appears that previous keystrokes (or repeated keys by holding keys down) build up in a buffer and get read and responded to later. If I don't press a key for a few seconds it usually goes back to normal, but it sometimes takes a few minutes.
I have not had the problem when using a keyboard encoder connected to the standard keyboard input but have heard of others having the problem with keyboard encoders connected to USB.
You might try playing around with the keyboard key repeat properties to see if it has an impact.