You don't need to uninstall the game in order to install Windows 7 on your computer. When you install the operating system you can format the disk partitions and implicitly erase the game.
But if you want to uninstall it before the Windows 7 installation, you can try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys or toolbars in your system.