Using Explorer, navigate to the C:\windows\system32\Macromed directory.
If you see a file named FlashUtil10a.exe, double-click on that file to start the install. If that doesn't work or that file isn't there, look for the file named Flash10a.ocx. If tat file is there, go to Start > and type:
regsvr32.exe C:\windows\system32\Macromed\Flash10a.ocx
If you see a box that said it registered successfully, Flash should now work, depending on what keys you deleted from the registry.