Knowing the default Windows password could be very useful for times when you forget our password or you need one to access a special area of Windows.
However, there are ways to accomplish the things you wanted to do with a default Windows password without actually having one.
For example, there are ways to find your administrator password, or any password you may not know, which you can then use in place of that fabled default Windows password.
Many of these solutions are specific to the operating system you happen to be running:
Note: The above links, and this discussion as a whole, only really applies to a standard Windows installation, usually on a single home PC or a PC on a home network. If your computer is on a corporate network where passwords are managed on the server, these instructions won't work.