Thanks All!

The reboot did it (Yay Culpeper!). The avast system tray icon now appears correctly, and my changes (resident protection in the simple interface set to standard) now persist. I loath rebooting my machine, but perhaps I should try that before posting any questions in the future.

For future reference :

Windows XP Professional
Avast Version 4.1 Home

techie 101 : I was initially only trying to set the resident shield setting in the simple interface, not for any particular section (IM/P2P/Standard), since these options were not available to me prior to the reboot.

Thanks again to all!