I had same issue with breaking my desktop after installing lastest update.
the system booted up, but my desktop was all messed up, no system tray, though I was finally able to pull it up from it's hiding place, but all my networking stuff had gone missing.
I am running XP Pro SP2, Avast is only AV, also run Malwarebytes, SuperAntiSpyware, and Spybot Search & Destroy, and that's it, never heard of Threatfire, and have no other firewalls than that from MS.
I was able to restore to previous date from Safe Mode, when it rebooted I tried to FIX Avast from the Interface screen, but had the same issue on reboot.
I then went into Safe Mode and uninstalled Avast, and manually removed all references to "Avast" in the registry, removed the left over folders in all profiles and from the Program Files, and am defragging as I write this.
I was going to download the program and do a clean install, but think I will read here for a while to see if Avast has put out any fixes for this issue, or if anyone has come up with a solid solution.
thanks,
niteowlgary