Citation:
If you use login controls, they will automatically use the membership system to validate a user. If you have created a login form by hand, you can prompt the user for a user name and password and then call the ValidateUser method to perform the validation. After the user is validated, information about the user can be persisted (for example, with an encrypted cookie if the user's browser accepts cookies) using Forms Authentication. The login controls perform this task automatically. If you have created a login form by hand, you can call methods of the FormsAuthentication class to create the cookie and write it to the user's computer. If a user has forgotten his or her password, the login page can call membership functions that help the user remember the password or create a new one.
Apparemment c'est la classe FormsAuthentication qui faut utiliser, en particulier la méthode RedirectFromLoginPage dans ton cas.