Force reauthentication

[jchamber2010]

I'm creating a system that needs to be slightly more secure than the normal forums, IE. CANNOT be access via the normal cookie save. Therefore I've set out to find a way that when a user enters the system it asks them to enter their password again (to verify they are who they say they are), and then sets a session for say 15 minutes of inactivity; or when their browser closes (which ever comes first).

Here's my dilemma:

What I'm looking for is basically that box that pops up every now and then when you try to delete more than 1 post on your forums. The box that says something like "Please verify the authenticity of your request by entering your password below".

Any help in the solution of this problem would be apreciated.