I got the above error message (This website is designed to work with Microsoft Windows operating systems only) when trying to run Windows Update. A Mac, really?. To fix the issue, and get Windows Update working again I deleted the following two registry keys:

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\
CurrentVersion\Internet Settings\5.0\User Agent]

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\
Windows\CurrentVersion\Internet Settings\5.0\User Agent]

Once deleted, I rebooted the computer and all was working again.

If you want to see what your Agent string is check out:
http://www.veign.com/tools/reflection/
(look at Browser information)

Check out these posts too:

  1. Download: Windows SteadyState
  2. How Windows Update Keeps Itself Up-to-Date
  3. One Update to rule them all
  4. Two Windows 7 links for running XP mode
  5. A faster Windows XP? Here’s how!
  6. 2007 time zone update for MS Windows operating systems
  7. Get your copy of Windows 7 RC (legally)