I remember seeing something like this in the forums previously. It was also win98 related, having something to do with win98 shutting down and not saving settings.

The solution if I remember correctly was to set the sensitivity to what you need clicking ok , etc. and closing the window. Then right click on the avast icon and select Stop On-access Protection and Exit. Once you have done this reboot your PC.

Try that and see how you get on.