I've enabled WEP on my Linksys router but the T42 can't find it. I'm sure it's because i don't know where or how to do it on the laptop. Any ideas?

You must enter the same network key on the laptop as on the router. Once you are able to connect to the router, turn off 'Windows configure the wireless network' if you are using Access Connections.

Do you have the Wireless update rollup package for Windows XP: http://support.microsoft.com/?kbid=826942? It addresses several things that may be happening behind the scenes.

Where do i enter the network key on the laptop?

Select the Wireless Network Connection that you created and click the right mouse button and select properties. Select the wireless network tab, make sure the 'Use Windows to configure my wireless network settings' box is checked. The linksys network should be listed as a Preferred Network. Select it's properties and make sure the Data Encryption box is checked. Enter the Network Key that you copied from web page of the wireless security page of the wireless tab of your router (if you are already connected to the router by ethernet) or copy it manually.

If you are setting up Access Connections, you can use their wizard to create a wireless profile (which is not the same as the wireless connection in Network Places).