No need for a USB to USB cable to stream content to the Xbox 360, just ensure both your PC and 360 are connected to your router.
Now load Windows Media Player up.
On the Stream button check the box to allow other devices to play your media, the first time you do this it might be called media streaming, i noticed it changed when i enabled it.
The Window below should then appear, click the button to turn on media streaming.
Then allow the devices you want to have access to your media and press ok
Now anything in your Windows Media Player library should be accessible to the 360 (and any other computers if you allowed them).
On the 360's dashboard go to Music and your computer should appear and you can then play your music from there.
Hope that works for you, I could do a more detailed tutorial sometime if you have any problems.