if you need to log the use in you need to take the BackofficeMembership provider and call the ValidateLogin-Method on it like this

Membership.Providers[UmbracoSettings.DefaultBackofficeProvider].ValidateUser(USERNAME, PASSWORD)

if that succeedes get the user and add the login to the database

BusinessLogic.User u = new User(USERNAME);

Guid userContextId = Guid.NewGuid();
SqlHelper.ExecuteNonQuery(
                                      "insert into umbracoUserLogins (contextID, userID, timeout) values (@contextId,'" + u.Id + "','" +
                                      (DateTime.Now.Ticks + (_ticksPrMinute * _umbracoTimeOutInMinutes)).ToString() +
                                      "') ",
                                      SqlHelper.CreateParameter("@contextId", userContextId));

And then add a cookie with the context

HttpCookie cookie = new HttpCookie("UserContext");
cookie.Name = "UserContext";
cookie.Value = value;
cookie.Expires = DateTime.Now.AddDays(1);
HttpContext.Current.Response.Cookies.Add(cookie);

The codes are taken from the login.aspx.cs and BasePage.cs you can take a look yourself

Hope thats what you wanted :)

Toby