XP keyboard problem

I have a user who is having a problem with his keyboard. When he types a 2nd letter the system doesn’t respond usless he waits for 2 sec. also the “backspace” key doesn’t always work.
I have run 2 antivirus programs, adaware, spybot S & D to clean the system fixed for 3 days then right back. Running the program 2nd time doesn’t help. Any sug would be greatly appreciated. Bill…