To Install Kali linux in a computer, the only thing you must do is to download and save the kali linux operating system from the source page:
http://www.kali.org/downloads/ (This is the official page of Kali Linux were you have to download the corresponding one for your own type of computer, I mean 32 or 64 bits and save the downloaded file into a CD/DVD or USB)
It’s possible to take a lot of time to download the file because the server of this page is very bad, but depending the moment it can be downloaded in 30 minutes or so. I recommend to try to download it in different hours of the day to see if the server is better or worse working.
In my case I used a CD and I have downloaded the 32 bits ISO image version for my own type of computer but check for your own case.
Then you only have to follow this web page (in spanish) where it shows how to access the BIOS of the computer to make the computer boot the system with the CD and not ths installed system.
If you don’t know how to do it and you don’t understand spanish language, do a google search and for sure you will found how to run the BIOS and change the booting to a CD.
There’s another good youtube video for the same purpose (in Spansih too): http://www.youtube.com/watch?v=nKvJ0URLptQ
Take care about what you are doing because if you boot via CD and you didn’t save your system files, it’s possible to lose everything. Another option is to do a re-partition of the disk first and install the kali linux as your second operating system. Check out this link for re-partition with Windows: http://docs.kali.org/installation/dual-boot-kali-with-windows
Then when the computer boots through the CD, it will appear a menu that will bring you to the install steps and when finishes the system will be running.
There is an image of what you must see, depending on the installer or the system you download, but in all cases you will see a similar menu:
So that’s all you have to do to get the Kali linux distribution system running on your computer.