In Windows NT 4.0 and earlier, the Sermouse subkey contained configuration data for the Microsoft serial mouse driver. In Windows 2000, this data is moved to the device-specific subkeys of the Enum subkey, such as HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Serenum\...\<Device-name>\Parameters. (The actual path to the subkey varies with the device.)
However, when you upgrade from Windows NT 4.0 and earlier, some of the values in the Sermouse subkey are copied to the device-specific keys. For details, see the descriptions of the Sermouse subkeys and entries.