You seem to have another antivirus active (resident) in your machine, don't you? Which one? Maybe you will have trouble with more than one residente av.
If you just want to scan you system weekly based, desinstall all providers except the Standard Shield (Control Panel > avast! antivirus > Add/Remove > Change (choose providers).
After this, right click the icon and choose the On-line protection and disable the Standard Shield. When the program asks you about 'persisting' the option, answer 'YES'.
Anyway, I do not suggest any of these procedures. How much RAM do you have? Which is your OS? Disabling the av application is not wellcoming...