If you use a standard keyboard without media keys, or want universal access to Windows Media Player when it's in the background, WMP Keys is the perfect plug-in. It sets customized, universal shortcuts for playing and rating WMP tracks.
Close down Media Player, install WMP Keys, then open it back up and head to your Options. In the "Background" category in your Plug-Ins menu, enable WMP Keys, and hit Properties if you want to venture beyond the defaults, which generally involve Ctrl+Alt key combos.
WMP Keys is, best of all, a free download for Windows systems. Click this link to download WMP Keys from Microsoft.