•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.