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Oracle's installation utility has a nasty habit of leaving a lot of items behind. I wrote this up to document what I've found you need to do to uninstall Oracle 10g manually. This is specific to 10g and Windows XP. Removing 9i is very similar if I remember right, but who's using 9i anymore? ;-)
The first thing you should do, is go ahead and run the installation tool to do an uninstall. It probably will leave some things behind, but it's worth running. Then go through this list and remove anything it missed. This list is detailed enough though, that I believe even if you did not run the uninstallation tool, this would fully uninstall Oracle.
The most commonly missed item is to make sure you remove everything from the GAC because the uninstaller doesn't appear to. I get to that later, but if you wanted to know the largest source of uninstallation problems, it's libraries remaining in the GAC.
After running the supplied Oracle uninstallation utility (which may or may not do some or all of the following):
Stop any Oracle services that have been left running.
Start->Settings->Control Panel->Services
Look for any services with names starting with 'Oracle' and stop them.
Run regedit and delete the following keys (some may have slightly different names in your registry):
HKEY_CURRENT_USER\SOFTWARE\ORACLE
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\Application\Oracle.oracle
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\OracleDBConsole
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Oracle10g_home
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\OraclService
Note that the services control panel will still show the old services until you reboot.
Delete the Oracle home directory
C:\Oracle
Delete the Oracle Program Files directory:
C:\Program Files\Oracle
Delete the Oracle Start Menu shortcuts directory:
C:\Documents and Settings\All Users\Start Menu\Programs\Oracle*
Where * indicates the name of your install. Look for and remove all Oracle directories from that location.
Remove Oracle refereces from the path. To edit your path go to:
Start->Settings->Control Panel->System->Advanced->Environment Variables
Edit both of the environment variables user PATH and system PATH. Remove any Oracle references in them.
Remove Oracle.DataAccess and any Polic.Oracle files from the GAC which is at:
C:\Windows\assembly\
There, now your system is Oracle free. If you are installing a new instance of Oracle (and not just an additional DB) I recommend you do this before any new Oracle installation.
Note: I used several other websites and searches as references when developing these steps, but I was unable to find any steps that covered all of the items for Oracle 10g so I assembled them here.
