I'm a AutoCAD operator been using CAD for more than 12yrs, AutoCAD can get very mix up saving in Autosave *.sav in the normal default temp directory.
To try and keep the *.sav file well away for the normal MS window temp directory, please follow my instruction.
1. Create an C:\Autosave directory on your HD which every your AutoCAD is installed on what drive.
2. Go into AutoCAD and click on the tools pull menu and click on options.
3. Pick the files Tab next to display tab on your left (Files) and pick Support File Search Path.
4. Scroll down and look for Automatic Save File Location, change the Temp directory save location to C:\Autosave.
5. Scroll down and look for Temporary Drawing File Location, change the Temp directory save location to C:\Autosave.
6. Scroll down and look for Temporary External Reference File Location, change the Temp directory save location to C:\Autosave.
7. Check to make sure you have complete instruction 4, 5 and 6 before you click the apply tab button and then click.
This should keep well away from the normal MS Window default temp directory, and you shouldn't have a problem between Avast and AutoCAD.
Autodesk cannot explain this problem and why it just the way AutoCAD been setup at the default installation of AutoCAD.