See here some supplemental information.

Before installing Oracle 11g database I deleted the registry entry HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE completely. So it's not a registry problem.

Actually the registry entry HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE only contains the entries "KEY_orahome11g" and "SYSMAN". "KEY_orahome11g" is the Oracle home from Oracle 11g database.

In the documentation I only found a section "Preventing Conflicts With Other Oracle Homes" but it only applies to Linux and Solaris, NOT to Windows.

Any other suggestions?