I dont quite understand the question. Do you mean installing backtrack on a computer that already has windows or mac os x or another linux distro already running on it? If so then yes you can instlal backtrack on other os. you just need to download the iso from the website, burn it to either a usb or cd. boot into the cd (different on each computer/mac)
then backtrack will startup wait for it to boot and run the installer. You will need to partition your hard drive dont forget. thats also different on each os!