As DavidPostill said - if you are using a company computer (or any computer on a company network) - your IT Admins can see everything you do - providing they have monitoring in place.
Regardless of what you are up to - your internet traffic goes out through their firewalls, proxies and routers. Your internal traffic goes through their switches and routers.
If your IT Dept has the ability to port-mirror, gather proxy logs, monitor firewalls, then everything you do can be watched.
If what you are doing is really that secret that you don't want your IT dept/boss to know - then don't do it at work.