Uninstall Oracle

1.Uninstall all Oracle components using the Oracle Universal Installer (OUI).
2.Run regedit.exe and delete the HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE. 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 obvious which ones relate to Oracle.
3.Reboot your machine.
4.Delete the "C:\Oracle" directory, or whatever directory is your ORACLE_BASE.
5.Delete the "C:\Program Files\Oracle" directory.
6.Empty the contents of your "c:\temp" directory.
7.Empty your recycle bin.

Manually Deleting Database
C:\>sqlplus sys/password as sysdba
SQL>startup mount;
SQL>drop database;