Okay, I have checked, and all of the Halo 2 servers ARE indeed still up and running, many of them populated.
So if you are able to pull any error messages off of your logs, that will help narrow down the trouble.
ICptJackSparrow:
Trouble Shooting and Error Log Collection
And now, for people who are still not able to get through trouble shooting this, we would like to see some error logs, heres what we'd like to see:
- Event Viewer
- Click "Start" > Search > Type without "quotes" > "eventvwr" > Press Enter
Expand the "Windows Logs" Tree
- Save the application file
- System information:
- MSINFO32
- Click "Start" > Search > Type without "quotes" > "msinfo32" > Press Enter
- Click on the "FILE" Tab > Save
- DXDIAG
- Click "Start" > Search > Type without "quotes" > "dxdiag" > Press Enter
- Click on "Save All Information'
- Please provide screen shots of any errors you experience, as pictures make things significantly easier to explain.
-
download: http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
after it installs you will see a lot of info about the processes you are running, with this program open attempt to start the game, when error pops do the following:

Hit Search, take a screen shot post it here.
Results shown for functioning Halo 2 on Windows 7 64 bit look like this:

(thanks to GFWMod for these)
Now, if you can post your results up, we can all have a look and see if we can't narrow down your problem.
I'm inclined to think a recent change to your system reset your firewall. You may want to make sure the proper ports are forwarded on your firewall and router to be certain you are indeed connecting to LIVE.
ICptJackSparrow:
Make sure that you have the proper firewall and port settings to allow your game to access the internet:
Port listings for Games for Windows - LIVE
If you do not use the Windows Firewall program, you can still open the ports that are used by the game when you configure the network. To make sure that these ports are open, or to open these ports, contact the network administrator or the Internet service provider (ISP). If you are the administrator of the network, see the documentation that is provided with your networking software to determine how to open these ports. The following list explains the port configuration specifications for the game:
- Port forwarding must be enabled on the UDP port 80.
- Port forwarding must be enabled on the UDP port 3074.
More on this subject can be found HERE
Full list of other Ports that you may need to open:
TCP 80
UDP 88
TCP 3074
UDP 53
TCP 53
TCP 443
Other MULTIPLAYER PROBLEM information can be found HERE
Basically, let us know anything, big or small that might indicate why you are not able to connect to LIVE.
Oh, and Wendschlag is right, for some reason the family subscription to Xbox LIVE does mess up some things. BUT if you were playing fine before, I'm still leaning towards something else having changed on your system.