Microsoft has a firmware update for the keyboard/remote/IR-USB Hardware. If you have one of the first one out they didn't work with the 2 IR senders and had a buffer over run problem. The symptoms of the buffer overrun are keys that appear to be stuck and repeat.
My keyboard was a MCE Edition v1.0 . My original firmware was 5.1.260.2825 7-1-2001 my firmware after update is 6.0.5308.17 6-21-2005. To check your firmware version you have to plug the receiver into a working XP Media Center system. Look in Systems / Device Manager / HID devices for the version.
Firmware Update Method #1
To update firmware you have to plug in the receiver into a working MCE system and use Windows Update program to check the system. If it works this will flash your MCE receiver if it needs it. If you fail on to method #2
Firmware Update Method #2
To update firmware you have to install Update Rollup 2 for Windows XP Media Center Edition 2005 in a working MCE system and this will flash the MCE hardware if it needs it. If I remember right Rollup2 also goes to the web and updates with a version newer than 2002.
Vista may also work to do a update but I haven't seen any documentation there. Important! Make sure to check your firmware version and date before and after updating to see if it took. Google MCE firmware update.