If that doesn’t resolve it, you may be asked to do a full update:

  • avast! Virus Definitions Update - Manual Download VPS Update (virus definitions, vpsupd.exe, approx 31MB). - Save the file to your hard disk, locate where you saved it and double click it and that should carry out the virus definitions update.

Note: Ignore the file name in the images of the page that is displayed after clicking the download link, they are for the main program, not for vpsupd.exe which will be downloaded.