When you say UNIX, you mean Linux no? as of true UNIX for PC platform you will find SCO or Solaris...
What do you know about linux/unix ?
I suggest you start using something like a live CD/DVD to figure out what you want once familiar, then go in installing as unless you use a cd to boot from, installing most (if not almost all...) on external drive, you will still have to install on internal disk a dual boot software...