When my application opens too many windows the taskbar groups them into one button. Each window has its own icon, but the grouping icon is the default "unknown"-kind icon.
How can I set the grouping icon?
The taskbar group icon can be set in the registry on a per application basis.
For explorer, this would be here:
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Applications\explorer.exe]
"TaskbarGroupIcon"="C:\Windows\Explorer.exe,13"