Internal Pop-ups

OS: XP
IE Version: 6
Symptom: User is experiencing pop-up windows when browsing the internal
network. Internal network is not advertising based and has no scripting for
pop-ups on any page.
Cache, cookies and internet files were deleted. No 3rd party applications
running that would cause pop-up banners. User has no external web pages
open in the background that could be causing the pop-ups.
Were could the hidden element be hidden that is causing these pop-ups even
when browsing a non-pop-up network?

Cap