It sounds to me like a computer issue, meaning you don't have admin rights over your computer & you may have installed into the wrong user account somehow. Or perhaps you installed as an admin but are now logged in as a different user but don't have full rights to access LW's program & folders. Do you think this could possibly be the case?

Out of curiousity, can you log into your modem? Try typing http://support.dlink.com/faq/view.asp?prod_id=1091 into your browser & see if you log into your modem or not. I have instructions for the D-Link DFL-300 which is not exactly the same as yours but probably similar. Your modem may use an internal NAT firewall in which case you should open a port for Limewire (LW.)

Which computer firewalls do you have? Is that XP firewall you're having problems with? There are alternative instructions posted by somebody for how to do it with XP's firewall. I'll hunt it down if you need.