A customer ran into the following error dialog when installing the 1.0 Framework:

      Error 1904 Module C:\Windows\System32\mscoree.dll failed to register.

      HRESULT -2146232576.  Contact your support personal.

The customer had the 1.0, 1.1, and 2.0 Framework installed, then uninstalled, then reinstalled and eventually got themselves into a weird dirty state.

They finally ran the clean up tool but setup failed when they tried installing the 1.0 framework.

After debugging their machine, it turned out the following registry key was leaked:

      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\policy\v2.0

I asked the customer to delete that key and the 1.0 framework then installed successfully.