Hi, Wecome to the Avast Forums.
Usually avast does a memory scan after boot and this should only take a few seconds. This is a default action and I would advise against disabling it.
If this is after your first reboot after installation? Then this is to ensure that you have a clean start state and should only happen once, unless of course you change a setting to do a check on the next boot.
I'm sorry not to be able to give a more detailed answer, this would require a more detailed question.
My advice would be to have a browse through the Avast Help file - Windows Start button, All programs, Avast, Help.
HTH - David