I'd try another keyboard first. If that works, try to repair the alt-key.
Kees
I got a problem - pressing f4 works as if I pressed alt-f4, simply pressing f4 closes any application. I need this button to use a certain program but I can't because of this 'bug". Anyone knows how to change this?

Chowhound
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GameFAQs
GameSpot
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TechRepublic