One instance; Run at startup; Save the input data

I have a small app: 3 input fields and 2 buttons.

What i don't know how to do:
- how to i make the app start in the background every time the OS ( Linux or Windows ) starts; what is the best solution at this time

- what would be a good choice to save the data from the input fields, so the next time the computer starts, the fields are repopulated automatically; i'm thinking to write all the data in a text file, but i also have a password there, so it's not nice

- limit the application to only one instance; if the user tries to open the app, the already running instance will be brought to the foreground

- use autostart
- see QSettings and encrypt your password (QCA e.g.)
- see Qt Solutions there is an example of an application which could only be started once.

As far as i can see, autostart works only on Symbian.
I will try to put a script in /etc/init.d for Linux, but i still don't know how to do it for Windows.

Thanks for your reply :)