Assume you already have Boot camp 2.1.
Check for firmware updates to your access point.
Try using different encryption settings on the access point. (Delete the network saved settings after each switch, via the control panel as below)
If that fails, go back to your original setting on the AP and try it on the laptop end:
If you view the Properties for your wireless network connection, there is a tab that retains the saved settings for each network. If you find your network, you can try changing the encryption settings. In some situations, XP uses the wrong kind of key (TKIP vs AES, etc.). Try each possible config and see if you get anywhere.
As consolation, my c2d 2006 MBP won't talk to one of my APs in XP. It connects, then drops, without transmitting.