In Win XP go to control panel, network connections, and right click on local area connection. Then choose status, support, details. Write down the IP address, subnet mask, default gateway, and DNS server info and then go to the Squeezebox and go thru the set up networking routine and choose "specify a static IP address". Copy the info you got from your computer exactly except the ip address which should be higher than your computers. So if your computers IP is, for example, 192.168.1.101 make the address for the SB 192.168.1.103. Make your routers address 192.168.1.102 and you should be in business. If you have other computers on your network check their addresses first to make sure that there are no conflicts (same address).
Do this and your only other hurdle should be configuring your firewall.