I suspect I ticked a "scan the machine for xxx at startup" box when installing Avast.
That's probably it. That will cause it to run a boot scan, which fits your description. (Looks a bit like the chkdisk scan.)
Hopefully it will just do that the once; once it's up and running you (should) have to command it to schedule a bootscan - a series of deliberate actions - before that will happen again. (The bootscan is a way of removing nasties that can't normally be deleted when Windoze is running. Handy tool, if you ever should need it.)
Re the VRDB, this takes several minutes and is visible by the animation of the second Avast icon. (The icons can be merged.) You can set it to generate VRDB when the computer is idle, which is how I've set mine. Takes a while to complete, but then only runs from time to time, I think following a Windows Update where the files concerned may have changed.