Evo ovako stoji na Microsfot sajtu:

1.If you have installed Windows Media Player 10, you can roll back to the version of the Player that was on your computer before installation by doing the following:

2.In Category View of Control Panel, click Add or Remove Programs.
Click Remove a program, and then do one of the following:

If you are running Windows XP Service Pack 2, at the top of the list, select the Show updates check box, in the Windows Updates section, click Windows Media Player 10, and then click Change/Remove.
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To je što se tiče deinstalacije, a evo odakle ćeš da skines stariju verziju
http://www.oldversion.com/Windows-Media-Player.html

Inače taj sajt ima starij everzije svakojakih programa