Two Macs, only one fully autheticates with MS proxy... any suggestions?

Question

*+The short version:+* I have two Macs with identical settings, but after installing Security Update 2007-007 one will play streaming MPEG audio in iTunes (amongst other things) and one will not.
*+The long detailed version:+*
I'm having a weird network authentication problem with two Macs that have identical setups. Both are behind a crappy corporate Microsoft ISA proxy server that has driven me nuts for years. They are both running identical systems (10.4.10), both have identical network settings and both are bound to Active Directory.
One Mac connects to the internet fine, while the other usually prompts for authentication or fails to connect with many apps. For example the Mac that works (let's call it Mac A) can connect to the iTunes Store and Microsoft AutoUpdate, while Mac B does not. In iTunes I would get the message “iTunes could not connect to the iTunes Store. You must log in to the proxy server in order to access the iTunes Store.”
The weird thing is that I could still listen to MPEG audio streams and access song names from the CDDB on Mac B (I have grown used to there not being true universal proxy support in OS X. Adobe CS2 Software Update will not work with a proxy that needs to authenticate, iChat has its own settings, and for a long time Mozilla with its built-in NTLM authentication was the only way besides IE that I could even get online before 10.4).
The problem for me is that after running Security Update 2007-007 I noticed that I can no longer listen to streaming music in iTunes on Mac B, but Mac A still works. That's when I noticed the other things that Mac A can connect to but Mac B does not, which is why this is starting to drive me crazy.
Things I've tried or checked to get Mac B to work:
-Set proxy to either IP address (Web Proxy (HTTP)) or Automatic Proxy Configuration, same problem on both.
-Both have identical Samba config file settings.
-Deleted all of my keychains and started fresh.
-I tried updating iTunes from 7.3.1 to 7.3.2, but no go.
-Both have the same time set (though they both cannot reach the time.apple.com server to automatically set the time. An error shows in the system.log).
-Tried creating a new test account on Mac B, but it has the same problems.
-Tried installing Authoxy 3.2.5 on Mac B to do the NTLM authentication, but I could never get Authoxy to connect. I get the following messages in Authoxy:
Fatal Error: unable to connect to talker socket. Errno: Interrupted system call
Couldn't open connection to proxy server. Errno: Interrupted system call
-I've tried running OnyX, Yasu and AppleJack to repair permission and clear out any cache that might be causing a problem.
-I've gone through the various log files to look for noticeable errors, but nothing seems to be coming up.
Network items that work on both Macs:
-Safari, Firefox, Dashboard Widgets, Software Update, Entourage, network drives
Another difference is that Firefox never prompts for username & password on Mac A, but does at startup every time on Mac B. The only way I can get around this is to manually create a Kerberos ticket and then Firefox will not prompt for authentication (Kerberos will not create a ticket automatically on both Macs, I read this was a bug with logging in to AD with an admin Mac account). But the Kerberos ticket does not help with my iTunes or other network problems.
If I've gotten all this to work, then why am I complaining? Because the streaming radio gets me through the day at work, simple as that.
I'm trying to be as detailed as possible so that I cover the bases and hope that maybe someone comes across this with a search and have found an answer or maybe have something new to try.
A big thanks for taking the time to read all of this and think it over.
P.S. I hate you Microsoft.

Answer 1

Aha! I've figured it out!
But not before resetting the PRAM and VRAM, re-installing 10.4.10 with the combo installer, and installing Wireshark to try and figure out what network requests were getting denied.
What did it was manually setting the IP address on the problem Mac to the IP from the Mac that was working. I came across this post (http://forums.macosxhints.com/archive/index.php/t-25049.html) where someone said the fix for them was to be in a certain IP range. I had previously tried renewing the DHCP, but I had to manually set the IP to the specific address to get it to work. I figured it had to be something on the server side since, like I said, both Macs had the same exact network settings.
Thanks to everyone that took the time to read my first post, and sorry about the typo in the title. It was late and I was getting frustrated.