Steps for uninstall Oracle from windows XP
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.
If you are running 64-bit Windows, you should also delete the HKEY_LOCAL_MACHINE/SOFTWARE/Wow6432Node/Oracle key if it exists.
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.
If you are running 64-bit Wiindows, you should also delete the "C:\Program Files (x86)\Oracle" directory.
Remove any Oracle-related subdirectories from the "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\" directory.
Empty the contents of your "C:\temp" directory.
Empty your recycle bin.