Go to Add/Remove programs in the control panel and you should see Windows Media Player 9 there, remove it and it should default back to what you had previously.
you cant uninstall wmp9 from winxp.
1)use system restore,like preet2u said.
2)install windows again(not formating)it will copy over the windows files.you need to reinstall some drivers like the video card and video codecs after,but your files and programs will be safe.
You most certainly can uninstall WMP from Windows. If the above do not work (WMP9 may be listed as viewpoint player instead of Windows Media Player) From control Panel > Add/Remove Programs > Add/Remove Windows Components > Uncheck Windows Media Player and click Next and follow the steps.