If you have a single port whatever does it need you to log on when you start ... or is it connected all by itself?
(did you have to enter an ID and password into it when you first set it up?)
if it does the logging on ... it's a router.
if it's a router ... what you need is a switch or hub (with only two pcs it doesn't matter which.
http://www.amazon.co.uk/s/ref=nb_ss_w_h_/202-9 745456-3611053?url=search-alias%3Daps&field-ke ywords=network+switch
the magic word is UPLINK ... if it's got it ... you just need
straight cat 5 network cables (3)
1 from router to switch ... 1 to each PC.
this puppy is a decent price ... and does have it
http://www.amazon.co.uk/Netgear-FS605-Platinum -Unmanaged-Switch/dp/B000087H93/ref=sr_1_2?ie= UTF8&s=electronics&qid=1208729241&sr=8-2
if no uplink ... you need 2 cat5 cables +1 cat5 CROSSOVER cable
either way ... your ouno puerto becomes threeo puertos
(you loose one connecting to the other deviceo)