How to manually remove Oracle 9i in WIndows

• Uninstall all Oracle components using the Oracle Universal Installer (OUI).
• Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE key. This contains registry entires for all Oracle products.
• Delete any references to Oracle services left behind in the following part of the registry:
  HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Ora*
  It should be pretty obvious which ones relate to Oracle.
• Reboot your machine.
• Delete the C:\Oracle directory, or whatever directory is your ORACLE_BASE.
• Delete the C:\Program Files\Oracle directory.
• Empty the contents of your c:\temp directory.
• Empty your recycle bin.

At this point your machine will be as clean of Oracle components as it can be without a complete OS reinstall.

Remember, manually editing your registry can be very destructive and force an OS reinstall so only do it as a last resort.