Some variant of this should work: http://support.microsoft.com/kb/314481
Just go to the registry key described there and find the right subkey, then delete it (make sure it's the right one first - you don't want to remove the wrong one.)
Also if it's a 32-bit app (likely) and you're running 64-bit, then look here instead:
HKLM\Software\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall