czaval
I would not recommend you using an INI file.Today you have more comfortable formats to store data in for manipulating it later.
You can use the XML file for storing the username and password, so it is very easy to store and retrieve data from it.
You can you a simple 'MS Access' database file with a single table for storing user login details.
Check the option of using the cookies, but you must be ready for loosing the data in the cookies because user can in any time delete the cookies.
Anyway, remember to think about the encryption if data is sensitive.