A Trojan is an email with an attachment, that if you open the attachment, it will attempt to take control of your PC. We do not recommend you open any attachments unless you are expecting an email with an attachment from that individual, and you know them. You should be suspicious of emails from friends and family with catch phrases like "You have to see this", or "Try this", or "Really funny", or any generic phrases like that.
The IT department WILL NEVER send out an email to our users, asking you to run an attachment, without contacting you first. If we have a program-fix we wish everyone to run, we will send you an email directing you back to our Intranet Web site where you will be required to login and download a program fix from there. From time-to-time, one of the IT staff may work with you one-on-one and may email you something to install, but you will know that ahead of time so you will be expecting it.