You will have to undo all of this to play any of your other games! Steam will crash or not work if it is not set back to normal!

Open task manager (ctrl+alt+del -> Task manager), and go to the details section, then scroll to find steam.exe and steamservices.exe

Right click on Steam.exe and find "Set affinity.

Once you click on Set Affinity, click on all processors and then limit them to 2 cores. This should restrain Steam to a state that allows prototype 2 to run.

Repeat the same steps from set affinity to steamservice.exe and then Prototype 2 should work! Have fun!