Vista - Adding a Laptop to existing Wireless Network

Firstly, I appreciate that the 3 PC's in this scenario are running Windows XP but this is fairdinkum to only place I trust for the correct information.

Basically, I want to set up a latop in a small business (same as if it was at home I presume) to share an internet connection with an existing Wireless network.

The current set up is a destop PC running XP, ADSL line running to a wireless router (Wincomm) , destop connected by cable to router. Laptop upstairs connected wirelessly to this set up.

I have an XP Laptop, for it I bought a rather cheap USB Netgear Wireless G USB 2.0 Adaptor (WG111v3). I can pickup lots of secured and unsecured routers but not the one as described above (I know what the workgroup name is)

Basically I have forgotten how to set up XP ( gee, I can do it if it was vista! )
My gut feeling is I have the wrong Wireless Adaptor for the third XP Laptop.

I'm not sure if I really want to join the network as such as the XP laptop I want to add has shared folders for when the XP comes home and connect it to the home network.

What are my options?

Thanks for your help