Set avast services to manual load:
Start > Control Panel > Administrative Tools > Services
I suppose you have to have four avast! services:
C:\Program Files\Alwil Software\Avast4\ashServ.exe (avast! antivirus service)
C:\Program Files\Alwil Software\Avast4\aswUpdSv.exe (avast! Update Service)
C:\Program Files\Alwil Software\Avast4\ashWebSv.exe (avast! Web Scanner service)
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe (avast! e-Mail Scanner service)
Well... you could make some mess with these changes, besides the fact you won't have resident protection.
Are you intending to use other antivirus at the same time in this computer?