Hi Pieterro,
Welcome to the Microsoft Community.
I understand you are having issues playing Cossacks: Back to War on Windows 8.
One reason you could be facing issues with the mouse could be because the game was not designed for Windows 8 but for Windows 95/98.
You can try running the game in compatibility mode and check if it works fine
a)Right click the game executable file and click "Properties" and then click the "Compatibility" tab.
b)Check the box "Run this program in compatibility mode for” and select Windows 95/98 operating system from the drop down.
c) Click on ‘Apply’ and click ‘OK’ and run the file to install it.
If this does not resolve the issue you would have to contact GSC Game World, the developer of the game check with them if anything else can be done.
Please feel free to use our forum for any other questions you may have.