Computer does not recognise device. When hooked up through USB port, windows does not recognise

Question

When hooked up through USB port, windows does not recognise device. My system is running windows xp sp2, 512 ram. The unit works fine through my laptop running the same operating system and acts as a mass storage device. i can view and transfer files back and forth no problems.i try to connect my mp3 to my computer to charge or to download on to it but nothing comes up on the computer