Keybord shift key mismatch

Hi,

I have this problem that the shift key characters are changing without any warning and go back to the original again after a while. Minutes or longer. Normal typing and CAPs are Ok.

One of the mismatched shift key characters are opening parenthesis that will become closing parenthesis (>).
I have this problem on several computers now (ace laptop, dell laptop, medion) . I dont believe the reason is the antivirus, however on all three computers AVAST is installed. One of them has AVAST pro version while the others have the home version intalled.

They are all running XP as operating system. and have the american Keyboard
Further on all three is firefox 3.0.1 as browser installed


regards,

Jan