Icon Disappears from TB when minimized

I have a peculiar situation on a Windows ME machine. When a running programm is minimized for instance Winword the Icon of the program in the Task Bar disappears. ALT-TAB also does not show any thing but CLT-ALT-DEL shows the program as running.Can’t seem to find any reason for this beahviour. Could this be a virus. Anyone who can shed some light on the matter.