Perhaps the "oraInst.loc" file (which contains the path to the inventory location) is missing or is in the incorrect location.

It should be located in the following path depending on the OS:

1) On Unix/Linux:
/etc -- or --
/var/opt/oracle
and always a copy under the $ORACLE_HOME

2) On Windoze registry:

HKEY_LOCAL_MACHINE->SOFTWARE->ORACLE->Key: inst_loc