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:https://www.reddit.com/r/Steam/comments/5h5z2w/steam_bootstrapper_not_responding/
Can also be antivirus blocking it.
Some people have fixed it by running the Steam client in compatibility mode.
Some people have fixed it by uninstalling Steam and everything except the steam\apps folder, then reinstalling.
Here's a thread from 2013https://steamcommunity.com/discussions/forum/1/828939978486814243/
Nobody knows for sure what "fix" might work with your particular computer.
You just have to try the different possible solutions until you find one that works.