installing a cat5 plugin to a computer

I am not sure what it is called so I will explain what I am trying to do.

I have a D-link 704up router.  I have a speedtouch modem connecting it to the internet via dedicated phone line.  I have a two systemax and one dell computer hooked into the router and everyone is doing fine.  I have one spot left on my router and I have a 2003 Systemax computer that I want to connect to that spot but it doesn't have a place to connect the cat5 cable.  I have tried searching newegg and thetechgeek and tigerdirect for a part to fix it but since I don't know what it is called I can't find it if it exists.  Thanks for your help.

I went to the thread on how to set up a network and it said each one needed an ethernet card and I even found one that was a usb ethernet so it would appear all that I needed to do was to plug it in to the computer and then plug the cat5 cable into it and that computer would be networked too.  

That's pretty much it, and the beauty of a USB ethernet device is that you don't even have to open up your PC.

How old is the Systemax PC?  Most PCs seem to have onboard ethernet these days.

After looking, I either bought it Dec 2001 or Dec 2002 not 2003, but I am pretty sure it was 2001.  It replaced my 1980's Tandy.  I ordered it from Tiger Direct and it came with windows xp.  When I bought it everyone was telling me I way overbought on the hard drive.  Friends told me 10 gb were plenty so I bought a 20

Thanks for your answer.  I have ordered one and soon we will all be on the internet together.

Okey dokey.  Come back if you have any problems!