nedo - On my XP machine (which I keep updated with the POS hack), Avast is now at 17.5.2302. After one of Avast's recent sub-updates, in Troubleshooting, the recent "nested virtualization" subcategory under "Enable hardware-assisted virtualization" disappeared; there is now only "Enable hardware-assisted virtualization". So that is apparently normal for Avast on an XP machine.
(However, on my Win 7 Pro 64-bit machine, I still cannot turn on XP Mode unless I UNcheck "Enable hardware-assisted virtualization" in its Avast; that has never been fixed. This remains so even though the "nested virtualization" subcategory still exists in Win 7 under "Enable hardware-assisted virtualization" with both checked.)
I no longer have a problem getting Avast's updates on any machine. However, a long time ago, I programmed my router to use the OpenDNS IP addresses for DNS lookup. That's an added safety step that everyone should take. You might Google for "OpenDNS in router". I also run the OpenDNS Updater on my XP machine as a startup item.
Good luck.