software acting like shift key is stuck

I'm having an issue where various programs are acting like the shift key is stuck down. For example, Firefox will treat CTRL-TAB as CTRL-SHIFT-TAB and everything I type is in caps. Sometimes if I bang on keys and minimize/maximize enough times this weird behavior will stop; but usually I have to restart the program. This only seems to happen to one program at a time. If it's happening in Firefox, I can go into another program I'm running and everything is working normally. And if it's happening in another program, Firefox will be fine.

I thought it may be some issue with the Accessibility Options, but I've turned all that off and still have the issues.

I'm running XP SP2 and use antispyware and antivirus programs regularly.

Thanks for any help. This is happening 5 or more times a day and is really a problem.