Use XP/2000lite from www.litepc.com and that will do the trick...
If you have XP you could look if you have one of those restore points from before ya installed WMP but I really wouldnt recommend it cuz Ive had only trouble with it when I used XP cuz it only restore some files and some dont so it can really mess things up..
I used to uninstall it manually if I wanted just to install an older version..
You just have to get rid of the reg-keys and some files that WMP looks too see if it is installed..