The fact that the WSC doesn't recognise avast shouldn't stop it updating, avast should check for updates automatically when you first connect to the internet and every 4 hours if you remain connected.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
How do you know they are infected ?
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here the URL in the Address bar of the VT results page.
