Download WDK ([http://www.microsoft.com/downloads/details.aspx?FamilyID=2105564e-1a9a-4b f4-8d74-ec5b52da3d00&displaylang=en]) and install it.
Under each directory there is buildall.bat file that will build the driver for different platforms. First build VirtIO lib (in Virtio lib) and then any needed driver. Currently the buildall.bat configured for 7600.16385.0 version of DDK. If you are using different version set DDKVER environment variable to this version.
You can also build the drivers by executing specific build environment from WDK and then running "build -cZg" command in appropriate driver directory.