I am not sure why I got this. I think I accidentally pressed some sort of key combinations and my build output is like this:
1>------ Build started: Project: test, Configuration: Debug Win32 ------
1>Compiling...
1>main.cpp
How to remove the prefixing '1>'

Weird build output
biscuitlad
Félix
Under Tools->Options->Projects and Solutions->Build and Run there is an option to specify the number of parallel builds. If you set it to 1, the output lines are no longer prepended with the build number.
The actual number of parallel builds is either this setting or the number of projects in your solution that is to be rebuilt, whichever is smaller.
Brian
David Rochat