Telnet activation on your pc isn't bad but know that, having telnet on is like leaving a port open to hackers and can be easily exploited if you are not cautious. You have to make sure you use a strong password or disable root login remotely

TO activate it,

go to control panel and click on programs and then programs and features. To the left hand side of it, click on Turn windows features on or off.

Then a box would pop-up with some features. All you have to do is to click in the boxes where there are provisions for telnet client and telnet server. And ok it.

You can go into the other person’s computer by typing cmd in run then hitting the enter key then typing telnet followed by the person’s ip address and then enter. Here you can totally control his computer.