You may be able to hide it from your co-worker (by installing it in a wierd directory (i.e. C:\windows\system\aim"), deleting the AIM program group from the Start menu (and running AIM.exe manually, by going start->run->c:\, and clicking up to the AIM folder, then doubleclicking AIM.exe) and turning off auto-load (by default, it will load up on boot, pop on your screen, and put an icon in the tray. You need to turn all this off).
However, you will NOT be able to hide it from your IS people, and you may not even be able to connect from work due to firewall settings. I've personally blocked AIM usage at several of the companies I worked at, it's not too hard (just block connections to *.oscar.aol.com).
Of course, if you've got friends in IS, you can just install it somewhere on the network, and map a local drive to that location. When your co-worker logs on, the drive won't be there, but when you do, it will (because drive mappings are stored in your local profile).