hi gtodave!
to switch on the avast! AV again just rightclick on the avast-icon in the tray and click on start on-access-scan (probably has an other name)
if that doesn't work it should start again after restarting the PC.
how do you know that you have a virus if you haven't got an activated antivirus???
i would recomend to run a boot-time-scan to find the viruses you are infected with...
yours
onlysomeone