ISA Proxy identifies old user when new user on that PC

A user surfs the web via an ISA Proxy server and is always identified by ISA as a previous user of that PC, even after rebooting the system and properly re-authenticating. Describe how this can be corrected so that the user is identified properly by the ISA proxy server?

This is a question I have to have answered within the next 2 hours.
I am allowed to ask for help… so I came here. I don’t have a working AD/ISA network to test upon and have exhausted searches throughout the net after working on this until nearly 3AM this morning!

PLEASE HELP!