How do I do a manual uninstall of Oracle?

The six-step process to remove all things Oracle from a Windows machine:

A. Delete the Oracle services: In the registry, go to \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services and delete everything that starts with "Oracle"

B. Remove other Oracle stuff from the registry: Go to \HKEY_LOCAL_MACHINE\SOFTWARE\ and delete the key ORACLE

C. Reboot

D. Delete all the Oracle software from the directories where you installed it

E. Delete the Oracle software inventory: Delete the directory C:\Program Files\Oracle. You must do this no matter where you installed your Oracle software - the Oracle installer automatically writes information here.

F. Delete all shortcuts from your Start menu.

G. Remove the Oracle directories from PATH Environment Variable.

To simplify cleanup in the future, I'd strongly recommend you install your Oracle products in one or more virtual machines.

Have a look at:

http://www.oracle-base.com/articles/misc/ManualOracleUninstall.php

Basically, it comes down to:

Remove all you can with the installer. Remove Oracle keys from the registry. Remove the Oracle directories from your computer.

With (of course) the requisite reboots thrown in as required ;-)