Ubuntu 12.04 LTS.
Every computer in the house has Ubuntu on it. My Mom's laptop, My Dad's computers in the office, (one is mostly used by my Mom), my My Dad's lended laptop which he hardly ever used has Ubuntu 10.04 LTS. I installed it, because it was failing, Linux brought it back to life. It has 10.04 LTS, because it is really old, and can't handle the latest OS. The rest of the computer have Ubuntu 12.04 LTS.
My Dad's main office computer, runs Windows as his main OS. His main computer, has two HDD's, one with Linux and one with Windows. The only problem with Linux, is that his accounting application (Simple Accounting) isn't compatible wtih Linux, and you can't import that data into any other app. I've tried. Also, the scanner doesn’t work, but strangely, the printer does. The scanner and printer is in the same machine. It's a printer, scanner, and fax combo. Everything else works.
Out of 6 computers in the house, only one that Windows exists.