Only these are avast's:
ashDisp.exe - MS Start-up entry - msconfig, startup tab.
ashMaiSv.exe - Windows Service - Manual Start - Starts if you have the Internet Mail provider enabled.
ashServ.exe - Windows Service - Auto Start
ashWebSv.exe - Windows Service - Manual Start - Starts if you have the Web Shield provider enabled.
aswUpdSv.exe - Windows Service - Auto Start
The main process being ashServ.exe, this would be the one scanning files at boot, to disable scanning, you would effectively need to disable the Windows services and msconfig startup entry, then manually start them which is probably a lot more hassle than what you have at the moment. Not to mention leaving you vulnerable.
I would suggest that you check exactly what programs are starting on boot, there are many that add themselves to the startup list when they don't need to start until you require them, media players being one such example. Disable those that don't need to start and avast won't have to scan the files that they open.