All our computers have Windows XP Pro Sp3 and IE8 with all updates available. I've setup a test machine from metal to clean instalation and updates.
Problem 1: computer freeze after install avast antivirus: still happens even when computer has been finished installed and all updates done. Instead of doing it by pushing a network script I need to go over to the computer because It will freeze and I can't even VNC/RDP to it.
Freezes are nasty to sort out. Can I be clear, your fresh install test had no additional unnecessary 3rd party drivers (like the printer/display) installed? Could you tell us if all the hardware is basically the same or different vendors on affected machines (eg all the motherboards are the same)?
If it stops at exactly the same place every time on a vanilla XP install on common hardware without joining domains or anything, it kinda suggests a hardware compatibility problem rather than hardware fault. Its very likely the install is not completing properly and therefore causing all your other problems.
Problem 2 SOLVED: active desktop problem: From your idea I've searched a bit more to do it automagically :-)
Nice.
Active Desktop is basically useless and even Microsoft dumped it. It will also help your machines out resource-wise.
Problem 3: unable to print and printer port tab crashs explorer.exe: I have the latest drivers for the 2 printers we use Utax P-4035i and a Panasonic DP-180, both printers are network accessed.
I probably wouldn't even try to bother solving this until Problem 1 is sorted.
I'd strongly suggest logging a ticket/chat with support, since it isn't an isolated incident in your environment. In particular, I'd ask if you should try an earlier Avast build which I think doesn't have any of the new NG or Secure Virtual Machine stuff in it to test.
From other posts that I have read, XP and Avast don't always play out of the box, as @PCSFL experienced. May I ask why you haven't upgraded from XP now it is end-of-life (apart from the old hardware)?
