I have NOT uninstalled Avast, I have not reinstalled Avast, I have not edited my registry. I believe that if I remove or remedy the problem, you have nothing to troubleshoot and nothing to learn, so I'm letting this system remain as it is, so that I can help Avast get this right. If Avast is still interested.
With Avast (currently running 4.7.1029) fully enabled:
After booting WinXP64, the system reaches the normal user login screen and then suffers from freezeups, remains frozen until I reboot; second boot attempt usually passes, a few times I needed a third boot and never needed more than 4 boots to get logged in.
The freezing is not related to what the user does - it might freeze during an admin login selection or freeze during a normal user login; it might freeze before while entering a password or might freeze during password entry or (if I'm VERY fast) freeze after password entry but before the screen ever clears. I've seen the system freeze BEFORE I could click a user icon. For the one time that I tested the freeze duration, the freeze has lasted several hours (I then gave up and rebooted).
With MSCONFIG, I used selected startup, uncheck all Avast startup items:
System is fully normal.
You have asked me if I found any unpxxxx files in <avast>/data/log directory; I responded I have not seen any of those files.
I have returned to allowing all Avast items to startup, I have only warm booted, warm booting always allows me to log in. I need full cold boots to get the freezeup problem at all. Now that I have restored Avast to being allowed to start, I believe I need a few cold boots to resume the problem.