ubuntu is like linux..also its free to download.and you dont need to buy it. but you cant use win apps in it ..it does have some apps built in like,,a word processor spreadsheet,,presentation app. usally alot of users use it as a dual boot with windows. heres the link so you can check it out
http://www.ubuntu.com/ also if you go on youtube,,, and just put ubuntu in search and you can see it in action..ubuntu aero in mind blowing better than vista ,,heres link to watch.
http://www.youtube.com/watch?v=bYsxaMyFV2Y