How do I know if I have an onboard LAN?

There are two ways; you can examine your computer - if there is an on board LAN, then there will probably be an RJ-45 connector on the back of the computer. An RJ-45 looks a little like a phone socket, but with a larger hole. The other way is to use the computer software; assuming you're using Windows XP, hold down the Windows key (usually between Ctrl and Alt) and at the same time press the Break key. Select Device Manager, if you have a LAN connection on board, then you should see a Network Connections tab which you can open to see the network connections available.