Make sure that the XP firewall's configured to allow LW.
Firewall setup

Has the firewall icon always been there?

You should find out what modem/router (there might just be a satellite modem, or there could be a router too) the main computer uses & if anything's changed since your LW stopped working.

In LW go to tools > options > advanced > firewall config. Change LW's listening port to 64049 > apply. See if that helps.

I'll be honest with you...I know pretty much nothing about powerline networking