Here's an article that should address your issue.
270717 PRB: Cannot Register Msaddndr.dll When You Install Visual Studio 6.0
http://support.microsoft.com/?id=270717

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Kristyn Wagoner
Microsoft PSS
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> Installation of VS 6.0 on Windows 2000 professional fails
> with the message that it can not to register MSADDNDR.DLL.
> VS .NET was installed before but I removed it completely.
> Any ideas?