You can invoke devenv.exe /diff File1.txt File2.txt from the command prompt or, if a Visual Studio instance is already running, you can type Tools.DiffFiles in the Command window, with a handy file name completion:
You can invoke devenv.exe /diff File1.txt File2.txt from the command prompt or, if a Visual Studio instance is already running, you can type Tools.DiffFiles in the Command window, with a handy file name completion: