You need to set-up each computer's mac address up with the stb if you want to plug each into the stb. I'm sure marge or someone wil provide you the details on how to do that.
As for sharing it then you have a number of options you could use ethernet to the pc form the stb then install a secondary ethernet card in the pc and use ICS (internet connection sharing) though you would then need that pc on all the time.
Best bet is get a router, they are next to nothing nowadays and you could even splash out a few quid more and get a wireless one. That way if you get a laptop you can connect it up as well.
Wired router £15.00
Wireless router £26.00
hope this helps
hm