When you created the password, you had the option to set up a reset procedure. Use that procedure to reset the password. However, BitDefender doesn't prohibit the usage of your computer, meaning that it doesn't install into the logon screen to allow logging in or not. If you want to remove the software, go to Start > Control panel > Uninstall a program. Since it's an antivirus, the program might refuse to uninstall if you choose 3rd party applications (eg. Revo Uninstaller) .
If your product is registered, use the following tools to uninstall it from your computer. Any password it might have will be removed immediately.