Few question

1) Can you surf the web from both computer?

2) Does both computer have IP Address provide by a DCHP server from the Wireless Router?

If both answer are true, then, you just want to make a workgroup between a 2000 and XP machine. You need to install XP Network Package that come as an addons on XP Cd on the 2000 Machine. If this doesn't work install Netbios and make sure you're on the same workgroup.