After scanning AVAST 4.5 Home Edition, my OS is Win98 SE, I encountered an Illegal operation below as well as “AVAST encounters problem” (Intenal Error).
This happens after scanning of about 15 minutes and it happened every time I run it at the same time and location.
I’m not a Windows 98 expert neither a debugger one… :-
But, why don’t you try just simple actions and try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the popup window (Repair).
You must be connected to the internet while repairing.
If this does not help, can you uninstall / boot / install / boot again?
Do you have any other antivirus installed in your computer?
Did you have in the past? Which one?
Do you use a firewall? Which one?