I have two hard drives on my computer. One Windows 7 and one Windows XP. So I installed the Windows 10 Technical Preview on the XP drive. It went smoothly and somehow I still have the capability to boot the Windows XP installation along with Windows 10 or Windows 7. Note that the Windows 10 Technical Preview will expire April 15th.
There's a cool website devoted to this http://windows10hub.com/ but don't yell at me if you screw up your computer.
Windows 10 is like Windows 7 with the Windows 8 tiles. That was a good move. I wouldn't upgrade but when you have to buy a new computer nice to know it still acts like Windows 7.