PC-BSD is an easy to install Windows like desktop operating system. I installed it last night and am running right now. (How do I look? :) ) For those who might fear or moan about the learning curve for a BSD OS, this gives you a lot of the look and feel of Windows but it is still a full blown FreeBSD OS and you can do anything a standard FBSD install will do. I installed this on a 550Mhz PIII box that was running Windows and I feel it runs much faster than Windows did.
Another to consider is DesktopBSD. I have not tried this one but it comes recommended also.
The only differences I am aware of if PC-BSD tends to be a little further behind the OS versions than Desktop is. DesktopBSD installs software from the 'ports' collection while PC-BSD creates their own pre-compiled packages called .pbi files. These .pbi files might give PC-BSD an advantage that you know they will work when they're installed but the packages are older than the DesktopBSD software.
From what little I've read, you can't go wrong with either one so if you have a box you can try this on, give it a shot.