You want the key under HKEY_CURRENT_USER. Stay out of HKEY_LOCAL_MACHINE.

Under HKEY_CURRENT_USER you can delete the entire "5.10" branch. Things will work as if you just installed the program.

(This assumes you're running Version 5.10. If you're running 5.00, then you want: "HKEY_CURRENT_MACHINE\Software\COADE, Inc.\CAESAR II\5.00".)