Hi,
All my downloads are placed in a folder I named "1st_Base" and then I copy them to to their final destination. I leave all the original downloads that I decide to keep in 1st_Base for many months.
Thus, as I never acquire files by any other method (e.g. CD) I almost never do an Avast scan except for a right click context menu scan of 1st_Base. So if Avast has catered for any new infections since I downloaded it's a quick process for what is an almost 100% check.
As Avast checks my downloads in any event I set my downloader to open ClamWin so that I can update it then close or scan just the file downloaded.
Sometimes I do a pre-check using Dr.Web online (via IE or Firefox).
I use Noscript and WoT which in combination with MailWasher and Avast has, for me, been fully effective thus far. Hardly any False Positives and not a single infection found since I started using Avast in May 2008.
My regards