Don't forget that XP has it's own firewall, make sure that it's configured to allow LW (it's better to have the XP firewall enabled, rather than using no software firewall at all).
User Cant Connect - LimeWire

To find your modem/router, have a look at all the devices that plug into your computer & allow you to connect to the internet. You could have a modem & a separate router, or a modem-router combo, or just a simple modem. Look for boxes with LEDs...if there's no brand/model showing, have a look underneath

If this is a new problem, have you recently updated/upgraded your LW or updated your Java?? First thing to try is this: Delete LW's preferences folder when LW's properly closed. See point 3 here & also point 2 if you've used older versions of LW in the past (there could be a folder in either location).
Fixes for LimeWire
This will kill all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through LW's tools > options & write them all down, so that you can reset things later.

If you've recently updated Java, some people have had problems with Java 6 update 1. If that's what you're using, try the latest Java 6 update 2. Use the JRE offline installer & remove your old Java version(s) before you install this one.
Java SE Downloads

If that Java is no help, try this older version. Same deal with the JRE offline installer & removing other Java versions
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11

If none of those help, try this.
General Connection problems

If that still doesn't help, get back to us & we'll try something different.