avast! is running in High priority so it finishes the given task faster and releases CPU power to other things. Changing to Normal would only make the things worse.
Alwil programmers know why it must be on High priority.

For correct memory usage you have to enable VM Size column in Processes list (Task Manager).
Open Task manager,select Processes tab,click View and select Select Columns… in dropdown menu.
New column will appear next to the Memory usage one. Now check the VM Size column and tell us again how much is it using.