Maybe this can help a little?
Does your login program start before Windows loads all the other prog's (example. Taskmanager etc.)?
When it doesn't, you can add a string to the Windows Register (from within your app) that will start your program next time windows starts. It will load your program first of all (even before it loads the background of windows). When your program ends, windows will continue loading all the apps, etc.
Don't use an InputBox to verify the password and username.
Use your own form with 2 text boxes or something and a "Login" button (that will check if the username and password are correct offcourse :-)
Next, when they end your program WITHOUT a correct username and password (trough CTRL + ALT + DELETE for example) the program must exit windows. So they CAN'T login without a valid login. And when this occurs, you must add a function (or whatever) that makes a key in the windows register that next time the program starts, it will start again.
If you need source code from me, just tell (and give email address) and i'll send it to you!
Good luck