How do you completely uninstall Oracle9i from a Windows system?

1. Ensure you are logged in as a user with Administrator privileges.

Stop all Oracle services.

2. Use the OUI and uninstall products.
This is done by selecting Deinstall Products and
Oracle Homes - OraHome92 - [x] Oracle9i Database

Then select the [Remove] button.

3. Using Regedit remove all Oracle keys under:
HKEY_LOCAL_MACHINE\SOFTWARE
HKEY_LOCAL_MACHINE\SOFTWARE\Classes, remove all keys that begin with Ora or ORCL
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\Application
HKEY_LOCAL_MACHINE\SOFTWARE\ODBC remove all keys related with the "Oracle ODBC Driver"

Close Regedit.

4. Remove Environment Variables
Also remove from PATH all oracle references
Example: C:\ORACLE\ORA81\BIN;C:\PROGRAM FILES\ORACLE\JRE\1.1.7\BIN

5. Delete all Oracle Icons from: C:\Documents and Settings\All Users\Start Menu\Programs

6. Delete <system_drive>:\Program Files\Oracle

7. Remove any Oracle-related .INI files that may exist in the Windows directory.
Typical Oracle .INI files include ORADIM73.INI, ORADIM80.INI,ORACLE.INI,ORAODBC.INI

8. Reboot your computer.

9. Delete all ORACLE_BASE directories
Example: C:\oracle
Note: If you attempt to do before the above reboot it will probably show DLL(s) being used.

10. Empty recycle bin.