I would like to repeat the comment of Gary Day out here, because it saved the day for me:
I stumbled across a way to install SDK 7.1 without having to uninstall the later 2010 Redistributables!
First dip into the installer Setup\WinSDKRedist_amd64 folder and install WinSDKRedist_amd64.msi (I am on 64bit Win7 but hopefully this will work for 32bit as well)
Then launch the main setup.exe. The installer now recognises part of the SDK is installed and skips the check for later 2010 redistributables and you can select change option and add the remaining items you want installed.