This happened to me recently. Looking around I found that on XP if you upgrade to the latest version of Flash Player 9, then the old flash.ocx file hidden away (in Windows\system32\macromed if my memory is correct) gets wiped out & a new file flash9e.ocx (or falsh9.ocx - either way it's a new name) replaces it - a sort of e-cuckoo.
When I copied the new file to "flash.ocx" in the same folder everything came back to life.