I am trying to use an Arduino nano to emulate a keyboard. I just found that we have to install V-USB. Is there an other method to do it without installing something. Can't we use just a library.
I have no clue what V-USB is but you cannot emulate a keyboard with a Nano. A Leonardo or Micro is a much better choice as they have USB built-in. With these boards you can just upload your sketch and the board emulates any HID device you like.
V-USB stands for "virtual USB". It's a bit banged USB implementation that can be used on the microcontrollers that don't have native USB functionality. The most common example is those digispark ATtiny85 boards that don't need a USB chip. It's a pretty hacky solution that I think is a bit unreliable. So it is likely possible to do this on the Nano but I agree with pylon that the best solution is to just use a Leonardo, Micro, Pro Micro, or MKR Zero to do this. That will be so much easier and also probably more reliable.