Installing on Systems That Already Have Oracle Components
Perform the following steps when other components exist on your computer:
- 
                     Log on as a member of the Administrators group for the computer on which you want to install Oracle components. If you are installing on a Primary Domain Controller (PDC) or a Backup Domain Controller (BDC), log on as a member of the Domain Administrators group. 
- 
                     Delete the ORACLE_HOMEenvironment variable if it exists. See the Microsoft online help for more information about deleting environment variables.