I had the same problem installing the windows SDK 7.1, vcredist_x86 installation failed although when i looked at the unistall a program tab of the control panel no VC 2010 x86 redist was installed. I also had the windows 7.0 SDK installed. It turns out, there is a registry key describing if and which version of the vc 210 x86 redist is installed:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\VC\VCRedist\x86
The key is Installed
Mine was at 1 even if the uninstall tab did not show it, i set it to 0 it did not solve the problem, but deleting the whole
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\VC\VCRedist\x86
did.
Hope it helps, I recommend you backup your registry before doing that.
I find it deeply astonishing that Microsoft would release a SDK with such a problem.