In Visual Studio go to the Tools->Options menu item. In the options dialog open Projects and Solutions->Build and Run. Change the value for "maximum number of parallel project builds" to 8, or whatever number you want (I believe the default is the number of cores on the machine).
For C++ projects in Visual Studio 2010 you can also enable "Multi-processor compilation" from the C++ General project settings. In VS 2008, the VCBuild.exe helper takes care of this already (see documentation).