Rather than running without a firewall, turn the XP firewall back on & configure it to allow LW
User Cant Connect - LimeWire
Or, if you'd rather, use the Norton firewall (make sure XP firewall is off) & configure that one to allow LW.

Update your Java. This version's the latest update.Download the JRE ofline installer & remove your current Java version(s) before installing this one.
Java SE Downloads

Some (not all!) people have had problems with the latest Java updates. I'm using the latest one & it's fine. If that latest Java gives you trouble, use this older version instead. Again, use the JRE offline installer & remove any other java versions before you install this one.
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11


Try deleting 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.

From what I can find out, that modem has only recently been offered to AT&T customers. Did you have any connection poblems with your previous modem? I can't find much info about it - not even a user guide! So I don't know if UPnP's enabled by default.
The info below says that the modem has a firewall. I can't find any info about port fowarding. ..look in the user guide that came with the modem. Or could call your ISP & ask about that (also ask whether there's definitely a firewall).
New Motorola Modem « Connected Home 2 Go

For your movie problem, use the forum search to look for codecs...you'll find heaps of links to codec packs & alternative players.