avast doesn't alter anything, windows manages the file system information.
Where are you getting this last accessed value from ?
I use XP Pro and there is Date Modified and Date Created in windows explorer (I believe the Last Accessed values aren't displayed by default). If you view the files properties you will see a last scanned value, but by accessing the files properties you automatically cause windows to change the last accessed date and time value.
I have just set the View option to display last accessed values, I then did a scan of that area and the last accessed values didn't change.
Why do you ask ?