"Zorin OS is a multi-functional operating system designed specifically for Windows users who want to have easy and smooth access to Linux. It is based on Ubuntu which is the most popular desktop Linux operating system in the world."
It has a number of different desktops that look like various other O/S's...
"The Look Changer lets you change your desktop to look and act like either Windows 7, XP, Vista, Ubuntu Unity, Mac OS X or GNOME 2 for ultimate ease of use."
I looked at it briefly a while ago and it looked good, but I didn't use it regularly as I didn't need it at the time. I'll probably load up my old Xeon system with it after April 8th.
I've no affiliation with them, but I thought it may be of interest.