I use Norton Antivirus 2008 on an HP desktop with Vista Home Premium OS (with Vista Service package1)
I have my quick scan set to scan after downloading updates, which I set updates for every 4 hours.
Sometimes I will notice a quick scan that scans a good number of files (at least once a day) and takes maybe 45 seconds
But other times I notice a 2-3 second "scan" after an update with no files scannned.
Is that because the update did not warrent a scan? Maybe the update did not contain virus defs?