Why does an ethernet usb adapter work on laptop but not main computer?

[totallylost]

Hi i am asking this question on behalf of a friend. He has a combined router and modem which he his trying to connect to his main computer. There is an ethernet cable to connect it to the computer but his computer does not have an ethernet card so he has an ethernet/usb adapter instead. When he plugs it into his main computer the relevant light on the router and the light on the usb adapter DO NOT come on, but if he takes the adapter out of the main computer and plugs it in his laptop both of the lights on the router and the adapter come on..
has anybody any ideas why this may be happening, both he and i are baffled as to why it works in one but not the other. Both are running on XP.
Gratefull for any suggestions. Thank you

[YeOldeStonecat]

I'd make sure Windows is updated..especially service pack 2 for XP. USB drivers were updated with that service pack.

Then ensure latest drivers for the USB NIC are installed. Check device manager to make sure it's loaded properly and not disabled. Then check in network connections..should see a LAN connection listed for the USB NIC.

[mccoffee]

some ports are powered usb some aren't too could be part of it