In the interests of research I just removed avast from my system and installed NOD32 (as a 30 day free trial).

Nice program.

Memory - the obvious stuff - two entries using about 30Mb (VMsize).

No noticeable affect on the usability of my system.

Restored my system back to running with avast.

Memory - the obvious stuff - 4 entries using about 41Mb (VMsize)

No noticeable affect on the usability of my system.

The memory size difference is a nit on my system (2Gb).

Given free versus annual fee and that avast has kept my systems virus free for several years (ie the entire time I have used it) the choice for me is an easy one.