1. You don't tell the server OS. That might make a difference.
2. It runs Outlook? That's a client program, not a server program.
3. "not able to clean the disk". Why?
4. Tell the statistics of the disk: total space, free space.
5. Most 'full' disks are full with rubbish and data, not with programs. So cleaning off programs generally helps less than cleaning rubbish and managing data.
6. "Can anybody tell me what programs I have on the c-drive?". No. But you can yourself using a not very advanced tool like Windows Explorer and Control Panel.
7. It's unusual for a machine to power down if the only problem is that the disk is full. There might be other problems with it.
8. A somewhat crude way to solve this problem: backup your data (if you haven't done so), format, reinstall everything (or use the image you made for this server) and restore the data. Be sure the backup is 100% readable or you'll lose data.
9. How's your security?

Finally, advice your boss to hire a part-time professional system manager or contract a third-party company to do it.

Kees