If your solution consists of .csproj or .vbproj, it is already a build script with MSBuild as the build engine. MSBuild comes with .NET framework, not with Visual Studio.
Edit:
You can launch MSBuild from the following locations:
%windir%\Microsoft.NET\Framework\v2.0.50727
%windir%\Microsoft.NET\Framework\v3.5
%windir%\Microsoft.NET\Framework\v4.0.xxx
Starting with version 3.5 of the tool, you can specify the target framework for which you want to build by specifying the "ToolsVersion" attribute on the project element.