DO NOT use registry cleaners! See this:
http://aumha.net/viewtopic.php?t=28099

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"Mark Dee" <MarkDee@discussions.microsoft.com> wrote in message
news:A7A3724D-B1C3-4A66-B615-33059CFB4787@microsoft.com...
> Do you have a registry cleaner ? I had a similar problem and it's now
> fixed.
> I used registry mechanic from pctools.com.
>
> I tried rolling back to a systme restore point and it sort of worked but
> then a lot ofother stuff started breaking. Since sorting out the registry,
> everything's been ok.
>
> It seems to be tha automatic updates that screw up the registry so much.
>
> "Lynn" wrote:
>
>> Suddenly media player just won't launch, even with run as administrator.
>> I
>> tried downloading media player for vista but it won't install saying
>> there is
>> already a Newer (?) version on the computer. So I tried uninstalling the
>> existing version but it doesn't even show on the list of programs for
>> uninstall. Not sure where to turn. This happened once before but a reboot
>> fixed it.