1) Buy a wireless router. Probably the best are the new 108mbps ones by Netgear, such as the: http://www.amazon.co.uk/exec/obidos/ASIN/B0006M5UQG/ref=br_cecns_1/026-3051622-7268453

2) Set this up as according to your isp (setup will involve a simple wizard.)

3) Put the router next to your main desktop pc, and connect them together with LAN cable (i think some comes with it).

4) To connect the laptop up wirelessly, you will need a wireless dongle/card. I recommend http://www.amazon.co.uk/exec/obidos/ASIN/B0006374PK/ref=br_ce_ts_3//026-3051622-7268453 for ease of use

5) Install the wireless adaptor on the laptop, and they will both connect to each other and the internet wirelessly! yay!