I didn't think Avast settings/troubleshooting/Enable hardware assisted virtualization was the same as not installing "Secure Virtual Machines" because it is neither greyed-out or unchecked on Athlon XP 3000+ machines which O know don't support "Secure Virtual Machines".
To be honest I'm not sure myself
I think it is a separate binary process to support Secure Virtual machine. Perhaps called AvastVBoxSVC.exe or NGservice.exe and possibly running as a Windows service (sorry don't have a machine in front of me to check).
My understanding is if the CPU didn't support the code, it wouldn't run regardless of the "Enable hardware assisted virtualization" tickbox state. The option I thought was to allow you to set the state only if it was able to run, but had no smarts around being greyed out etc.
This question might be more suited to the "Avast Free/Pro/IS/Premier" forum since that is where the codebase comes from, lots of discussion there on the subject.