If' it's just to check hotmail, you can use this site:
www.anonymizer.com
Or try to use a "CGI proxy". The browsing activity with a CGI proxy looks similar on the logs to browsing a normal webpage. Now if he catches you using a CGI proxy and blocks it, it means that the admin is currently watching you and examining all the websites that you visit. In this case you need to take a break so that he forgets you, or change your IP, since he cannot possibly watch all the IP's.
I'm not sure though if ISA will also log the MAC address or username, as I said before, I never worked with ISA.