Unity WebPlayer installation fail on Windows Vista 32bit / IE9

On a computer of one of our customers the Unity webplayer fails with his installation. I tried several things like: http://forum.unity3d.com/threads/126769-Public-testing-of-the-Unity-3.5.1f1-WebPlayer and How I do re-install the webplayer when it fails to work? - Questions & Answers - Unity Discussions and tried to install from the .cab file and to install older versions like the 3.4.2f3. Manaul installation won’t solve the problem. And everytime I uninstall the webplayer I reboot the pc, before installed a new WebPlayer.

But always in the installed programs the Unity Webplayer is 208KB instead of the 12MB - 15MB as normal. The computer runs on Windows Vista, 32bit with IE9.

Is there anything I can do to fix this?

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