Application writing to itself?

Does anyone know how an application can write data to itself? (To it's .exe) An example of usage would be to store a username and a password as for registering an application... etc.

Thanks

Here is an example on making a Active Self Extracting Exe, the author claims that you can store usernames and passwords in the exe as well.

Active Self-Extract

I think you would have to write it in assembler to be able to do that, or at least write parts of it in assembler, the usual Idea is to encrypt it and store it in the registry, there's a link called registry on the blue bar at the left of the page, check in there for all your registry needs.