I also found this on the web :

1. Stop all it's services (sid, listener, agent, and http).

2. Start the registry editor: Choose Start -> Run -> regedit

3. Go to HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE Delete the Oracle Key

4. Go to HKEY_LOCAL_MACHINE\SOFTWARE\ODBC and expand all subkeys and remove all keys under here which are related with the "Oracle ODBC Driver"

5. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services and remove all keys under here that begin with ORACLE or ORAWEB.

6. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\Application and remove all keys under here that begin with ORACLE.

7. Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall and remove any entries related to Oracle.

8. Go to HKEY_CLASSES_ROOT, remove all keys that begin with Ora or ORCL (e.g. Oracle..., ORADC..., ORAMMC..., OraOLE..., OraPerf... and ORCL...).

9. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Classes, remove all keys that begin with Ora or ORCL (e.g. Oracle..., ORADC..., ORAMMC..., OraOLE..., OraPerf... and ORCL...).

10. Go to HKEY_CURRENT_USER\Software\Oracle, delete this ORACLE key.

11. Go to HKEY_USERS\?\Software\Oracle, delete this ORACLE key.

12. Close the registry

13. Choose Start > Settings -> Control Panel -> System -> Environment tab

14. At "System Variables" click on the variable PATH in order to modify the value. For example, you may see a path similar to this one: C:\ORACLE\ORA81\BIN;C:\PROGRAM FILES\ORACLE\JRE\1.1.7\BIN

15. If an %ORACLE_HOME% was installed, remove this %ORACLE_HOME%\BIN path.

16. If JRE was installed by Oracle, remove the JRE path.

17. Check if there are any other Oracle variables set in "System Variables", ORACLE_HOME, ORACLE_SID, TNS_ADMIN, JSERV or WV_GATEWAY_CFG. If these variables exist, delete them too.

18. Click on APPLY and OK

19. Close the Control Panel window

20. Choose Start -> Programs -> Windows NT Explorer

21. Go to %SystemDrive%\WINNT\PROFILES\ALL USERS\START MENU\PROGRAMS

22. Delete the following Oracle related icons:

- Oracle Installation Products
- PRODUCT_NAME - HOME_NAME e.g.
- Oracle for Windows NT - Dev6i
- Oracle Reports 6i - Dev6i
- Oracle Olap Client 2.2 - Dev6i
- Oracle9i Lite
- Oracle - OraHome92

23. Go to %SystemDrive%\Program Files\Oracle and delete this directory

Note: In order to successfully delete all files, you may have to reboot your computer first, in order to clear Operating System locks on those files

24. Go to the temporary directory and delete all files and directories in here

25. Go to the drive where the Oracle software is installed on your machine and delete all ORACLE_BASE directories on your hard drive

26. Close the Windows NT Explorer

27. Empty the recycle bin Right click on Recycle Bin -> Empty Recycle Bin.

28. Reboot your computer.