depends how you install it--it needs a clean install so if you only have 1 hard drive then it will wipe it--you can install it in a virtual machine using virtualbox or similar
personally i have tried it in a vm and wasnt very impressed with it --though its early days yet --it just looked like windows 7 except for the metro bit which is really aimed at touchscreens
I used a spare HDD I had lying around, plugged that in, set it as first HDD in the Bios, installed the Developers preview and checked it out, once finished, switched the HDD order in the bios again and i am happily back to windows 7!
or just partition your hard drive... 20 gb should be enough
and install windows 8 in that partition
by the way just in case you dont know in this way you will dual boot
that way you dont need to find another hard drive
If You've never set-up a dual boot before, I would suggest running it in a VM first. If you like it, then go ahead and set up the dual boot. I set it up in a dual boot with windows 7 on an Aspire 9300 laptop, and after 20 minutes of using it, I was wishing I had just used a VM. But that's just me.