I use Ubuntu at work. Fast, no virus, free. However, it does need a little of getting used to. All the buttons are placed opposite to that of Windows - so when you are too quick to click, you end up closing programs.
One of my main work machines at work is dual booted with XP and Ubuntu because I need XP for statistics (software only available for Windows). Now getting the rest of the 10 PCs changed over to Ubuntu. Our IT guy is having quite a time...
Gutturalpiss is the expert here in Linux.