Added help to the Windows install section

When attempting to install the Windows 7.1 SDK it may error without reason. This can be caused by having prior C++ 2010 redistributables installed. Uninstalling these redistributables will allow the Windows 7.1 SDK to install successfully.

References:
http://support.microsoft.com/kb/2717426
http://stackoverflow.com/questions/1901279/windows-7-sdk-installation-failure