Fake “No Access” page displayed in IE6

I have 3 users (out of over 1000) that see a plain white page with “Please log into the network for Internet Access. See our Internet Usage Policy for more information.” when they try to access any page that isn’t on our local network. This message is not one of ours (all of our notification pages have our logo) and we’re all baffled by it. The users were able to access the internet without problems until yesterday. Our fear is that this is a virus or shady ActiveX app, and that we’ll begin to see more of these in time. Has anyone ever seen this?