I know this is a weird question, but I would like to know if there is a way to send a text message to a PC whose IP address I know.
For instance, I am downloading a file through torrent from a PC on the net.
Through torrent I can know the IP address of this PC, and I want send a text message to the person who owns this PC.
Is this possible?
The answer is No. You would have to send that through an open port on the recipients PC which a firewall would surely block (Windows Firewall, router, etc..). Even if it didn't get blocked - you'd need something to turn your message from zeros and ones to text - and display it on the users monitor. You'll need to write a software client to do that, and get that person to install it.
There are probably some black-hat techniques to do this, but again, you're talking advanced software and network penetration.