I've installed Avast on an older system, circa 2007, 2GB DDR 1, AMD 64 X2 4200 dual core.
When I installed Avast I did a custom install, deselected the IM and p2p shields and all the "extras" in the middle column.
I don't want any of it. Startup is reasonable in XP SP3, I would say there is about a minute of extra activity,
probably Avast searching/downloading and activating updated patterns.
Beyond that, I find the performance impact mostly unobtrusive; it behaves better than MSE (in my experience)
in terms of its general impact. (That may be not saying much).