I've googled a bit about your modem and what Ive read so far points at a general problem in regard to it:
http://www.dslreports.com/forum/rema...tj715+firewall
That led me to take a look at the TJ 700x Cable Modem: Specifications. When you scroll down that page it says: "Filtering: Ethernet, LLC and IP filtering, operator-configurable for all models"
http://www.terayon.com/tools/static_...cat_id=9.3.2.2
I'm not sure whether the brickwall comes form there, since you said that the problem occured just recently. You may check again the XP built-in firewall being still disconnected and you also may consider to remove the other firewalll.
Though you should not run without firewall, I reckon you configurate the XP firewall to allow LW pass through rather than running without one.
http://www.gnutellaforums.com/showthread.php?t=51361 