That is harsh! You have a very strict environment. Most likely they have blocked the ports typically used by the majority of instant messaging programs. They may even have filtering software that detects IM packets and blocks them. Sounds like where I work!
eMessenger will work, because you’re using regular HTTP traffic to visit the site, send and receive messages.
Another option that might work is Gush. It is a Jabber client. Like eMessenger, Jabber also uses plain HTTP communication to a server, which in turn translates those packets to instant messages and sends them across the appropriate network. The advantage to Gush is that once you setup a Jabber account (which you can do for free on their public Jabber server), you can connect to AIM, MSN, and ICQ (maybe even Yahoo… need to check on that one).