Heyyo,
Free, faster, more customizable (for some this is too confusing).
Linux is a labour of love really. You're gonna have to learn some commands n' such for this one. Picking up a big linux bible's always a good idea.
It's pretty cool, open office works great for a free office suite. (eat it m$ office).
Longhorn.. now... finally support changing mobos. Linux has had this for YEARS. m$'s just following in their tracks on this one. Lately it seems m$'s done more trailing than inventing.. that windows scanning thing that can optimize system preformance for gaming in Longhorn does sound good, but the minimum specs of Longhorn? sounds like you'll need that windows scanning program.
Now, Linux yes, as I've said, really customizable. Some distros are easier than others. Fedora's defiantley the easiest for us Linux n00bs. It does all the hard work for ya. The only stuff you need to do manually is get some ntfs packages for it is all really. If you wanna game, you can try compiling wineX from the CVS that way you have a pretty decent way to play games or wait for Wine to improve.. it's on its way.. or if you've got some cash lying around you can subscribe for cedega (I hate this idea.. you should be able to do a 1 time payment solution as well).
So yeah, obviously Windows is the best for gaming since everyone supports windows really. But once gaming starts to become more linux friendly? we'll start to see the m$ monster stuble.. that'll be the day eh?
Now macs. MacOS is pretty good. BUT, for Imacs? crap. crappy-mc-crap-crap. Mainly built for school's, and pristty girls who think it's "cute." They run like it looks, small and impotent. even customizing the theme onnit tends to crash the macos cause the low system specs can't handle it, lol. One great thing aboot macs is how efficient they are. They use a special achitecture for their cpus which is way better than our x86 architecture (which was designed for CALCULATORS.. not super computers. That's why you hear super computers with over 20 cpu's. x86 architecture is old, until there's a leap in that we won't see uber massive breakthoughs.. macs new deal with intel sounds like a step back in time to me.
Anywho, enough rambling, macOS is also better than windows cause it's more efficient. It's based off linux too. Now, just like linux, macs suffer from lack of gaming support, it's quite minimal. Unlike linux, macos costs money, so that's also a down side to it.