I keep a USB to Ethernet Adaptor in my tool kit.
As an example. When I have a problem that involves Wireless, the first thing that I do is plug this device to the USB port of the concerning computer, and plug the computer into the Router/Switch/Hub to make sure that the Network settings, email etc. working correctly. Then I switch back to Wireless.
They are cheap and work well.
The only problem is deceiving advertising.
Most brands advertise them as 10/100. However it is USB1 and the USB part is capable to run only 11Mb/sec.
When I asked why it is ?pushed? as 10/100, I was told that it means that it is compatible with 10/100.
There is one USB2 adaptor available, and it can runs much faster.
I did not see it yet, and it very expensive so for the time being I do not see it as a working solution.