Avast is the best! However, many times a virus may exist without passive automatic detection by Avast antivirus software. Only an active manual quick scan will detect the virus.
The Avast team needs to develop an optional scan on shutdown. This option will perform a quick scan on the active Windows partition everytime the user logs off and whenever the PC is shutdown or restarted.
Please please Avast developers. This is SO Necessary!
DrPCDr.ca
Welcome to the forum. 
IMHO, this will add an unacceptable delay to both system start and shutdown.
It isn’t anything that can’t be accomplished manually now if it was desirable.
i think if it’s optional then it can’t be unnaceptable … it’s all up to user …
it’s no problem with Pro version and it’s cmdline scanner …
SuperAntiSpyware Pro can be set to do startup & shutdown scans. SAS Pro isn’t an antivirus, but it does detect & remove trojans and other nasties.
The Professional version shuts down the computer after a scanning (optional).
But avast hasn’t an option to scan at every shutdown. Anyway, if you have the resident turned on, it will be a hard thing to support having to wait each shutdown scanning…