Try something just as a diagnostic, although it may be a way to fix your problem. Right-click on a blank part of the system tray. If Lock The Taskbar is checked, temporarily un-check it. Select Properties. On the Taskbar tab, there is a pull-down selection for Taskbar Buttons, which probably says Always Combine, Hide Labels. Select Never Combine, then click OK. See if the icons behave properly. If so and you want the stacking feature back, go back in and set the choice back to Always Combine, and click OK. See if the icons go back to proper behavior. Sometimes, just forcing it to re-save the settings will clear up corruption. If not, preventing icon stacking may serve as a workaround until somebody comes up with something else...