As already stated this is an old topic, and topic reply subscriptions have long since expired so these replies are likely of little use to the topic starter, or those who replied. However, I'm going to add my two cents anyway... Because I can.
We often see this question here, and almost always it's approached as a technology issue, when I think it's really a parenting issue. In my house I don't need monitoring software, because I am physically monitoring computer use. Our computers are always used in public areas of the house. You want to see what somone is doing? Look over their shoulder. No one is locked away in a room, or even has a private system -- they're all shared.
If our systems have a password, I put it there. Why on earth would a 15 year-old be allowed to put a password on their computer that prohibits access? Would you allow them to put a lock on their room if you didn't have a key?
Computers are just another form of communication. When I was young it was the telephone. I spent hours on the phone as a teenager, but I wasn't allowed to be locked away in my room where my parents had no idea what I was talking about. Did they record my converstations and listen to them later? No, they required I hold my conversations within earshot. Of course they didn't listed often, but I knew they could. It's really no different with computer communication. Whether it's IM, email, or video chat. If you need software to monitor computer use, reconsider your computer placement and usage policies.