First you'll need to remove Windows media Player 10. To do this, go start->control panel->add/remove programs->add/remove windows components (from along the left hand side), then scroll through the next window until you find Windows Media Player, uncheck it, and press next. WMP10 should now be history.
Next, you'll need to download an older version of Windows Media Player. Since Windows XP originally shipped with WMP9, that is the oldest version natively supported by the OS. You can try an older version if you wish, but there are no guarentees. To download an older version, just click here. Your WMP time machine should now be finished.
Personally, though, I wouldn't go through the trouble (and risk) of using an older version of WMP, as WMP10 has sports a newer, cleaner-looking interface and a few new features. If you have your heart set on bringing an older version back, I'd suggest clicking here to download Media Player Classic. It is a free third-party replica of WMP6.4, but unlike the older version, it still supports many of the latest features and does not require you to uninstall WMP10. I'd suggest giving it a try first and see if it fits your needs.
Hope this helps,