Using certain macros that work on Microsoft's Outlook, the Melissa program copied the first fifty email addresses stored in the program's address book and sent a message to each of those addresses. That message was personalized to identify the name of the sender. Now think of the mischief possible if some terrorist could trick your computer into sending an email to that address. To the computer law world there are some lessons. Yes, it is probably legal for your employer to use the office PC to record your actions.
Many supervisors do monitor email.
Many of those organizations have written policies that specify what, if any expectations of privacy employees may enjoy when using office supplied devices. You know that privacy is not assured. The recipient may or may not sell that data. You may or may not know that it will be sold. The software to secretly record voice conversations using the PC's microphone doesn't exist-yet.