Think about what changed since when it last work until now. Something
Odds are something physically wrong with keyboard. Perhaps someone
spilled something on or into it overnight. Prhaps the wire has become
disconnected. Borrow a keyboard from another computer and try it. You
may have to buy a new keyboard.
You don't say what you reinstalled. If it was XP, why did you think you
had to do that? You should not *reinstall* XP just to fix this simple
problem. Avoid doing this except as last resort.