When I was working they had a policy that said "no personal use of company computers", and they searched you disk frequently to make sure.
No outside emails.
no personal files.
no shopping.
no games.
no unauthorized web sites .
no use of search engines.
If you wanted to access an outside site for business reasons, you were supposed to get written permission from your boss first.
Obviously most people violated these rules frequently, and most people didn't get fired for it. But if they wanted you out, they had ample evidence of misbehavior.
MD