Get UserId from Table aspnet_Users

Hello,
I'm working with authentication mode = "Windows" where users entering the application are those that are part of AD. but to give permission to certain users to access certain areas, I'm saving these names "domain / name" a table aspnet_Users ASP, where it receives a UserId (uniqueidentifier). to emphasize that I am not using the membership table to track users, just use the above mentioned to assign roles to users table.
my problem is this. when used the membership table where the users were registered, I took the Id this way:
/ / get the id of the User Logged MembershipUser user = Membership.GetUser (); Guid userid = (Guid) user.ProviderUserKey; but not now as I use this table, only use the aspnet_Users table, would like to know how to catch this same id.