Error when installing Microsoft Mathematics silently.

Using this guide: http://www.itninja.com/software/microsoft/microsoft-mathematics-add-in-for-word-and-onenote/2-3147 I extracted the contents of the Microsoft Mathematics .exe file into a temp folder, and used msiexec to start an unintended installation, the problem that I'm having is this:

msiexec starts to run and works perfectly until it gets to 'Writing system registry values' at which point I am given the following error message.

"Could not write value AdminDisableSxS to key \Software\Microsoft\Mathematics\Standalone\4.0. Verify that you have sufficient access to that key, or contact your support personnel."

I have tried this on two separate machines, both running Windows 7 x64, one a fresh install and one existing installation.  If I run setup.exe and follow all of the prompts, Mathematics installs fine so the msi is clearly missing out on something but I can't figure out what...