If you need to know how to spy on a computer, I'll tell you right now that it's with software. But not all are created equal… some can spy on certain activities, but not everything. What's more is that most do not do so invisibly – and this is the key thing here. Invisibility and stealth are paramount to the success of monitoring computer activities no matter the reasons. Why?
For example, this is not how to spy on a computer; if you're trying to keep an eye on your kid while he's online to make sure he doesn't go off on some crazy bestiality porn site, it would do nothing if there's some blinking fireball of an icon pulsating in the system task bar. He'll clearly know he's being watched, and go off to a friend's house to check out some porn on his friend's computer.
Obviously, that is not how to spy on a computer at all. In order to do so effectively, you absolutely positively need stealth. You need to be invisible, or it just ain't happening, period. For this reason, when you seek out some software to monitor computer activities, it's got to run silently and invisibly… like the types used by businesses who keep tabs on whether their employees are indeed working, or stealing account information and client files for their own ends. These software programs record everything, every detail, instant message, every keystroke, and hides them on secret invisible files that then get discretely e-mailed to the observer who plays it all back for view.