Open Question: Why Does my Computer Say It Can't Find Any Device?

I keep plugging my MP3 in the usb and Windows Media player 11 and Rhapsody both say they can find a device.

The weird thing is, that my sister and I both have MP3's and the comp cant fid the device when we connect it to the usb

but my other sister has an I-pod, and the comp recognizes her device just fine

Does anybody know why this is happening?
And how it can be fixed?
Thanx!