If you're building a computer for a relative or friend and wish to avoid the cost of Windows 7 you might look into Zorin — the Linux OS' elements such as the taskbar, file system, start menu and desktop all work just like Windows. This combined with Zorin having the WINE Windows Emulator preinstalled means that the end user can install Windows programs easily.
I downloaded the ISO (around 1.14GB) and ran it from a DVD. Windows emulation worked well and although the familiar copyrighted logos are missing otherwise it looks and runs just like Windows 7 for average computing tasks.
In addition to WINE Zorun also comes with Play-On-Linux, another Windows emulator I'm not very familiar with. The OS also includes lots of standard open-source/freeware apps and there is a software centre that functions just like the Ubuntu software centre but with a slightly smaller catalogue.
You can also change the look of GUI from the "Look Changer" in the control centre to mimic Windows XP, Vista, Mac OS X, or a vanilla Linux GNOME interface. Zorin's Background Plus makes it easy to use a video file or screensaver as your background.
I like that it has the security of Linux built on the extremely likable interface of Windows 7. Depending on how well it recognises your devices. I had no problems running it on my Lenovo laptop, but your mileage may vary.