Igor,
Ok. I can live with it. I've just want to help you to find a bug.
I was able to reproduce such problem on 2 other computers with Win98SE with standard Windows install (no additional programs, only Win98 + avast 4.7 Home).
System resources are >85% free. The problem is appeared only if I select some directories to scan (third option on the right side of Avast scanner window). There is no problem if I scan local or removable drives (first and second option).
Maybe you have a glance on it in a free time. Just start Avast scanner, select directories to scan, close program, and open 'My computer'.
I think there is something wrong in dialog where user selects directories.
Regards,
Oleg