As far as I'm aware you are still able to do a custom install, changing install location and elements, etc..
There is nothing to stop you going back in and uninstalling components that aren't used. Windows add remove programs, Avast, Change/Remove, this opens another window and you click the Change option. You now have the various elements of avast with can be unchecked and OK your way out.
I would however, suggest that the minimum would be File System Shield, Web Shield, Network Shield, Mail and Behaviour Shields.
It doesn't matter if you have all shields installed as if they aren't scanning anything then it wouldn't be using any CPU resources over and above a minimal amount of RAM. If you aren't using an application P2P or Instant Messaging, then the associated shields would be dormant.
Avast5 is lighter on resources than avast 4.8, so what are your system specs, RAM and CPU ?