Windows Xp firewall, double-check your settings: Firewall configuration instructions (click on link)

WRT54G ... eek ... not the most reliable Linksys device on the market. They have many but that's probably their most unreliable & linksys are generally reliable! Check all the WRT54G links under the UPnP section of this link Connection problems? Check here first! (click on link); Check the link called "Linksys WRT54G - Fixed my problem, Followed these steps and it works! for a solution!" as that may be a part of your problem!

BTW it might be worth your bother to install an XP2 patch ... be it a windows one or a 3rd party one or both; see MS & 3rd party patches