The administrator is an account on the XP box. You will need at least a guest account and allow guest access.
You can use a terminal command, findsmb, which will return the address of all the windows xp or linux boxes using smb. This will also return the name of the machine, and the workgroup or domain it belongs to.
To get the network browser in the Finder to function, you must have SMB on in Directory Access, and have it configured with the workgroup name. Only computers with sharing on will show in the Network browser. The findsmb will show all computers on the network.