According to the Windows 8 Developer Preview download page, you can upgrade Vista or 7 to 8DP if you don't want the developer tools. But you must do a clean install if you had XP, regardless of if you want the tools or not.
So it gave me an idea that I'd just like to ask about: is it possible to dual-boot my XP laptop to make it run XP and 8DP, or do I need to do a clean install over XP?
(Note: I want to avoid virtualization because I don't like it.)
Simply create 2 partitions and install XP onto the first and 8DP onto the second.
I think you will have to do 8 second so the bootloader recognises the older version of Windows. Then when you boot up you will be able to select which OS you boot from.
UPDATE: (Taken from this question) 1. Yes, W8 supports dual boot, BUT you have to install it on another partition from bootable media (such as DVD or USB drive).
2. Do NOT install W8 from inside your OS by mounting the ISO, it will install over your primary partition WITHOUT giving you any other option (and without explicitly telling you what's going to happen).
You CAN do a multiboot Install without clearing anything
(This is important. Do not use the autorun installer. The autorun installer only allows you to upgrade your existing Windows installation and will not allow you to install to a separate hard drive or partition.)
sources. setup.exe and proceed as if installing Windows Vista or 7. 
Links:
http://www.ghacks.net/2011/09/14/how-to-install-windows-8-from-usb-key/
http://techspyre.com/how-to-directly-install-windows-8-without-cd-dvd-or-usb/
http://www.reddit.com/r/windows/comments/kf7ax/installing_windows_8_without_bootable_media/
