I've been having connection problems for about a week, and while searching through the forums has convinced me that my problems aren't unique, I haven't yet found anything that helps and I'm at the end of my rope.

Limewire worked fine for about two weeks, and all of a sudden it stopped connecting. The bars at the bottom of the screen are red with a tiny sliver of green, the firewall icon appears, and it won't connect. However, I've checked and rechecked Windows Security Firewall--made exceptions for Limewire and the ports, turned it off completely--and nothing works. I don't have another firewall program.

I have:
Windows XP
Wireless internet through Air-Pipe
Linksys WRT54G wireless router
Windows Firewall

I've tried some stuff like switching ports but I am SO over my head it's not even funny. Please help!

First thing to try is try deleting your LW Preferences folder whilst LW is closed; instructions here: Point #2 & 3 in Fixes for Limewire (sample image included at bottom of page.)

If that doesn't help .... your Linksys model is a reknown "lemon" model that Linksys makes. Their products are normally very good except for any model that has teh 54 number in it & the WRT54G is the worst of those. the 54g is unreliable in operation (behaves fine one day but not the next or even the same day.) Two options: 1. try port forwarding. 2. try the adjustments to the firmware suggested (this is a risky thing to try so be very careful.)
For Port forwarding, see section 3 (Port Forwarding) here: Connection problems? Check here first! (click on link). For fixing the firmware, you will find a link to it under the UPnP section (section 2) at the end of the Linksys listings of the previous link. Try port forwarding first. With Port forwarding, setting up the static IP is the first & most important step. This is where people generally stumble at. So follow instructions very carefully.