I'll have to do some checking under a Windows authenticaiton setup in the web.config but..... the basic idea behind the provider from day one, as I understand it, was to give the option for users to be able to use their windows credentials to login to DNN under Forms authentication. Once you switch to Windows authentication you're basically taking the provider out of the equation. I have a feeling that if you disabled the provider everything would still work the same in on your website.

That said, I do know that under Forms authentication, if a user logs out it won't automatically log them back in again. Part of this is by design (it may be necessary to log out and log back in as another user) and part of it is IIS itself (under authentication it creates a temporary cookie that lasts anywhere from 30 minutes to an hour that needs to expire before authentication takes place again).

Are your users still getting randomly logged out and then have to manually login? Or do they only have to login if they click the Logout button?

If it's the former I'll have to do some testing to refresh my memory on what happens (it's been a while that I just left a portal sitting there inactive) but I'm pretty sure on my portals it does automatically log the user back in.

If it's the later then it's as I said above, until the cookie expires it won't automatically log the user back in by design (the user clicked logout for a reason, generally to log in as another user).

If you are getting a Windows Authentication login, then DNN isn't asking for it, IIS is.  If the website is not listed (in the browser) as either an Intranet site or a Trusted site, IIS cannot automatically authenticate (unless you change the advanced settings), so it gives you the login prompt.