Unless the target computer had some sort of anti-virus protection, which will likely detect the attempted download, Back Orifice is almost completely undetectable without searching through the entire computer. Windows was specifically designed to allow system administrators to take control of the computer, but the designers apparently weren't thinking of this kind of attack. This is a product using a legitimate function of Windows. There are ways to use it in a positive way, and ways to use it in a negative way. The Windows operating systems were designed to be networked together.
Networks generally allow system administrators to remotely monitor and help users.
Back Orifice uses that same functionality, but doesn't let the computer user know that it's been installed. Outlining the Defense Becoming a victim of Back Orifice is actually somewhat difficult. Don't download something from an unfamiliar Web site, and don't open e-mail attachments sent from unfamiliar addresses. Log off of your account and unplug your modem. Then make backups of all of your personal files.