I might have answered my own question
Ignoring the fact Windows is imaged with sysprep, I did not use the VirtualBox clone feature to duplicate the VM guest and then move to another physical computer host. I was lead to believe you just needed to clone if running on the same physical host because the UUIDs would conflict without it. I just copied the VM files over and used "Add" to read the VM config back in.
Under normal circumstances this is not a major problem if MOVING the VM, but I intended to run both copies of the VM at the same time. I believe this might be what is getting me in strife (when software uses the hardware/SMBIOS UUID to identify the unique machine as I expect Avast is probably doing).
A more correct method would have been to use the Virtualbox clone feature, then move the newly created clone (which has a new hardware UUID generated automatically) to the new physical host. Alternatively I should be able to generate a new UUID and then use
vboxmanage modifyvm "myvmclone" --hardwareuuid <newuuid>
on the new VM make the SMBIOS environment unique, regardless of OS uniqueness.
Anyway I will try this out next week.