Depends on how many computers, if it's only 2 then heres what you need.
1. 2 Network cards, one for each PC, you may find these already came with the PC(s), Go into Control Panel - System - Device Manager and look under "Network Adapters" if you see a networking device you do not need to buy a network card for that PC.
2. If it's only 2 PC's you are networking, you can use a direct connection of CAT5 cable, available very cheap from your local PC Store.
3. Simply plug in the cables and it should be up and running.
Estimated price is around £10-20 for the cards and probably a fiver or less for the CAT5.
Asus A7N8X Deluxe - Rev 2.0 - Bios Ver. 1007
AMD Athlon XP-M 2200+ - @ 11x188 = 2Ghz (1.75Vcore)
ATi Radeon 9700 Pro (Almost as good as my old 5950U!)
Corsair 512MB XMS TwinX Low Latency DDR 400 - @ 188Mhz
3dmark03 Score: 5086