avast released a boot time scanner for both x32 and x64 systems.
It runs very early in boot process of course.

But, sometimes, users just want to see the interface, have the comfort of setting the scan settings with the GUI, etc.
avast at logon was a tool that holds the logon process (does not allow other applications to run in background) and load avast at the first time while logging in. In theory, other background applications are hold as well and won’t interfere in the scanning (speed).