Help making a E2 plugin to create or edit a XML file

Hi,
Please can anyone help me, I'm looking to create a little plugin to create a xml file or even edit one..
I think to create and del the old one would be easier .. 

I need a little popup box with that will say 

username 
password 

When they enter their username and password they click save and it will create a xml file in the keys folder.. 

Hope some one can help me