Windows Authenication and Userid's

Hi I normally use the built in membership provider and and forms auth and stamp user records with their userID using Membership.GetUser().ProviderUserKey. I'm now using Windows authenication, and can add users to roles, but how do I get a handle on the userid. It looks like .net under the hood creates and manages users, I can see them in the users table, but the above wont work because it is looking for the user id i the membership provider. Any Ideas Andy ...