Don’t blame it on Avast. Blame it on yourself and your son.

When it was detected, you should have enabled “remove at next boot when needed” and Avast would have handled it.

Other way to deal with it was, disabling the process and than Avast would also have taken care of it.

A deep scan with Avast indicated that this file was infected with w32.sdbot-194-B presumably picked up by my son when he installed Win XP and connected to the Internet prior to applying all the latest patches.
After installing any OS, always put a firewall and AV software on it before connecting to the net.