The computer should be scanned for viruses at least weekly. The best way to accomplish this is to setup a schedule with Avast that scans on a specific day and time. I usually have mine scan in the middle of night when I am not using the computer.
In addition, the software should be setup to scan email, file attachments and downloads from the Internet on a constant basis.
There could be several reasons that all the files do not defrag - the files are open, not enough disk space or they are files that can not be defragged (such as the Windows swap file). If it appears the most of the drive is defragged and only a few files do not defrag I would not worry.