In Visual Studio, how do I debug a project without building it

Please right click solution name and go to Properties -> Configuration Properties -> Configuration, clear the Build checkbox of the project.

In VS2013 it is: right click solution -> Configuration Manager, and clear Build checkbox of your project(s).

I found it convenient to create a new build configuration in order to do this. Right-click the solution in the Solution Explorer and choose Properties. From there, choose Configuration Properties / Configuration. Now choose Configuration Manager and create a new configuration as shown. You can create the build configuration as a copy of your Debug project:

Then, press OK and turn off all of the project build flags:

Visual Studio will have created a folder underneath the bin folder in the startup project with the name of the new configuration. Copy your pre-built files into there and run the new configuration.