I would imagine they are. You do have to be informed, I believe, but this can be as part of your terms of employment (it usually is) or contract or just being told. On some systems, it comes up as a reminder when you logon. 'You are reminded that the company's IT policies...' It doesn't have to be an explicit, individual notice.
If you don't believe that everything you're doing on the company's network is being monitored, you're a little naive. They have to cover themselves, in case of legal complications blowing up. EG abusive emails to colleagues.