Trying to intercept every browser launch probably requires monitoring the system processes. I.e., your app will constantly be running and whenever a recognized browser process starts, you spawn your login form. There may be system events to hook into (I am not sure), but How can I list all processes running in Windows? might be a good starting point to get an idea what's possible.
A somewhat different approach would be to register your app as the system's default browser. That way, you will be notified, can spawn your LogIn form and can than delegate to an actual browser.
I am not sure what ultimate goal you pursue, and whether either is a good solution depends heavily on whether there are more constraints to it than you mention explicitly.