Maybe you dont like the current WMP you have an want to Downgrade but get the annoying message you have a newer version??
Navigate to this key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MediaPlayer\Setup\Installed Versions
and change the values of the 3 subkeys to whats listed below (This makes it look like you have wmp9 and can then re-install 9/10 if you want to)
"wmp.dll"=hex:00,00,09,00,a4,0b,00,00
"wmploc.dll"=hex:00,00,09,00,a4,0b,00,00
"wmplayer.exe"=hex:00,00,09,00,a4,0b,00,00