MartinS,
the easiest way would to go for a wireless router (see the links earlier in the thread) and to shove a wireless card in each PC. This gives you the best security and means that you can move your pc around as well (should you wish).
however the costs are quite high, you would need
1
Wireless Router at £79
and
2
Wireless Network Cards at £36 each
Please note that these links are for kit that uses the newer 54 Mbit wireless networking standard, if you want to go for 802.11b (the older 11mbit standard) you could probably knock £20 -£25 off the cost.
Here I have shown Belkin as they seem to be the cheapest.
Hope this helps
Simon A