How do I completely remove Oracle 11g?

First, use Start -> Run -> Services.msc, locate all Ora* services and stop them.

The rest, from Manual and Clean Uninstall Oracle for Windows :

1. Uninstall all Oracle components using the Oracle Universal Installer (OUI).
2. Delete the HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE (and HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ORACLE on 64bit) key which contains registry entries for all Oracle products by using regedit.
3. Delete any references to Oracle services/components in the following registry location: HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/. Looks for key entries that starts with “Ora” which are obviously related to Oracle.
4. Reboot the workstation.
5. Delete the ORACLE_BASE directory. (i.e C:\Oracle)
6. Delete the directory C:\Program Files\Oracle.
7. Empty the temp directory.
8. Empty the recycle bin.

9. Remove any oracle entries in the PATH variable under Computer =>Properties => Advanced System Settings => Environment Variaiable => PATH
10. In the same place remove the ORACLE_HOME entry

Steps: Stop all Oracle services first.

1. START -> RUN -> Services.msc
2. Locate all Ora* services & click on STOP
3. Start -> RUN -> Regedit
4. Locate HKEY_LOCAL_MACHINE folder in the registry
5. Open Software folder
6. Delete the Oracle folder under Software
7. Open System folder in HKEY_LOCAL_MACHINE
8. Open CurrentControlSet folder within the System folder
9. Open Services
10. Delete all keys related to Oracle. Every key starts with “ORA..”
11. Close the regedit
12. Delete ORACLE_HOME folder
13. Delete the Oracle folder in Program Files
14. Reboot

I came here because of this message when trying to install Oracle 11g XE, after a previously failed attempt:

"Installer has detected an already existing XE instance on this system. Installation cannot proceed"

Of course I cleaned all according to various sources, including this one, but it kept complaining. It also did not show up in the list of installed programs.

I remembered a very old utility, that was shipped once with Office 2000 I guess. It is called "Windows Installer Clean up utility" (the original download is called msicuu2.exe). Download it from a trusted source, run it with admin privileges, and you'll probably find an orphaned record for Oracle in the list.

Click [Remove], confirm the warning and proceed with the setup. It worked for me.