The Steam bootstrapper app is the app that has to be running on your PC in order to install and play games (that you've obtained from Steam). You cannot play their games any other way on a PC, regardless of whether they were free or you paid money for them.
If you've installed Steam, you already have the bootstrapper. Try Googling the problem and you'll find many things can cause the bootstrapper error and it is not a new problem. Several possible fixes, including unplugging the gamepad: