Maybe 2.12 can't update 2.8 directly anyway. Are you sure there's not an intermediate version between 2.8 & 2.12 you have to upgrade to first?
If that's not the case, then Windows 8 probably will allow you to run the CPP2 2.12 setup.exe in Windows 7 compatibility mode - although I don't know how you do this on Windows 8. On Windows 7, you right-click on your executable in explorer and choose "Troubleshoot Compatibility" It's also possible you need to run the setup program with administrative privileges. Again, on Windows 7, this is a right-click pick on the executable itself. Not sure what you have to do in Windows 8.