have you tried tunneling through? are you allowed/can/will anyway
to install applications on your work machine?
Set up a simple proxy server and pass it through standard port that is allowed and connect it to a proxy server on your desktop at home or something like that if i remember rightly.
I had some limited wins of doing this at uni. Its no good for downloading, but should work for IM.
Ask whoever configures the firewall to open TCP port 25999 to cs.xfire.com - say it's for really important work-related stuff like letting your mates know what games you are playing.