Chances are it will be your wireless. Remember that win XP has its own firewall. See
Firewall configuration instructions (click on link)
1. Do this simple test for us. Using your browser go here:
http://www3.limewire.com:6346/ The page should say 'Your test worked!'. What happens when you go there?
Whilst Linksys devices utilitise UPnP which LW can also utilitise, some models (particularly the older ones) might not have this ability. Thus, forwarding a port might be the best answer. (Ethernet would be preferable to USB!)
2. Modem & router brand name & model numbers?
3. What version of Limewire (LW) & Java do you have? (Go to LW's menu Help>About LW...) Will show both LW & Java version.
4. Who is your isp provider?