As you can read on Lifehacker (here), and on a number of other sites, with a simple registry hack you can modify the way windows handles clicks on stacked apps in the taskbar.
Here is the tweak:
- Launch regedit.exe (Win+R, then paste
- Navigate in the left tree control to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
- Go to Edit->New->DWORD (32-bit) Value
- Name the value LastActiveClick
- Hit enter to assign the value and change it to 1
- Restart Explorer and you're good to go.
To restart Explorer without rebooting, open the Task Manager (Ctrl+Shift+Esc) and end the Explorer.exe process. Then create a new task (under "File") and paste "explorer.exe".