Hey Javier -
When you uninstall Server, we actually leave behind certain configuration information to make it easier for people to quickly get back up and running. In your case, you don't WANT the configuration information to stay around, because you want to start completely fresh.
The steps in this article tell you how you can do a "complete" (including configuration information) uninstall:
However, to address your question, you can reset the admin password without an uninstall/reinstall. Check out this thread:
that worked well. Thanks Christopher!