The answer might depend on operating system you use.
On MS Windows, you'd open "Services" (from "Administrative tools") and stop all Oracle services that are running. I wouldn't know how to do that on other operating systems.
Anyway: you are installing it in a wrong order - should be
1. database
2. Reports
3. Forms
For more details see
David's message on OraFAQ Forum (believe me, he *knows* that stuff very well).