What should I do if we are facing the issue while installing the oracle and there is one instance is already exist on system ?
•Uninstall all Oracle components using the Oracle Universal Installer (OUI). •Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE key. This contains registry entries for all Oracle products. •Delete any references to Oracle services left behind in the following part of the registry: HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Ora* It should be pretty obvious which ones relate to Oracle. •Reboot your machine. •Delete the “C:\Oracle” directory, or whatever directory is your ORACLE_BASE. •Delete the “C:\Program Files\Oracle” directory. •Empty the contents of your “C:\temp” directory. •Empty your recycle bin.