Just for your information:
I was working with my system since version 9 on the same or at least similar problem. Always the
first boot of the day takes about half a minute longer than all the following boots before the avast! tray icon appears. First boot of the day ~60 seconds, rest ~25 seconds.
Until the tray icon appears the system is very chewy and unresponsive on first boot.
I tried repair, clean install, disabling everything from the troubleshooting page, (I don't have dropbox) no success!
According to task manager avast! is working like mad during the last 30 seconds on the firts boot of the day.
What is avast doing different on the first boot of the day, what is it chewing on?
I have no idea!