Once, I was nearly fired for using my employer's computer for free-lance work. Sometimes companies go overboard to prevent these kinds of abuses, compromising privacy. See Commandment 9 for more on this. Thou shalt not appropriate other people's intellectual output. Thou shalt think about the social consequences of the program you are writing or the system you are designing.
Companies similarly can intercept any e-mail sent through their equipment.
Morale can suffer if you don't inform employees of these practices beforehand. Thou shalt always use a computer in ways that ensure consideration and respect for your fellow humans. This is another good catchall. It's easy to preach to the wicked, harder to admit fault. All of us, guilty of human frailty, stray from the straight and narrow in small ways or large.