The password is stored in the registry. If you remove the appropriate registry entry, you'll clear the password and regain access to PTFB Pro. The usual caveat applies - do not touch the registry unless you have prior experience of registry editing.

For those who are comfortable with registry editing, proceed to the following key:

HKEY_CURRENT_USER/Software/TLHouse/PTFBPro/Options

...and delete the value named "Password".