So to throw my 2 cents in since most of the other answers seem to be pro-Mac.
Windows: Windows has a bad rap for being unsecure back in the day, Window 7 is much better since it has an entirely different kernel than WinXP. Vista even had a different kernel, but Win7 kernel is improved over Vista's. The reason that there are so many virus for Windows is because of security flaws within the system which lead to several ways to make hacks. Many of them have been fixed. Somebody said their windows takes like 7 minutes to boot up, haha mine takes maybe 2 or 3 minutes and yes there is a ton of software on it. I just don't let everything start up that doesn't need too. I recently have had only issue on windows is that there is a lack for free software, but there is quite a bit as well.
Linux: Linux is probably my favorite kernel to work with, whether I am running Ubuntu, Fedora, OpenSUSe, Puppy, they all have their pros and cons. All software is not free under Linux, but most task can be accomplished with free software. Linux can get virus especially since everybody wants a point and click system nowadays. I prefer to compile all my own software under linux since its possible and you get it specifically for the architecture I need. Also, I work in computational sciences so Linux is my favorite development environment, especially will all the compilers included. I do have the Intel compilers on a few machines which are amazing to use on Linux. I'd highly recommend them. They do have windows and mac versions, but I don't remember the last time I saw a Mac cluster. With the development of cloud computing and the online software to edit files, Linux will start becoming even more popular due to price, speed, security, and new GUIs. Linux can also read/write nearly any file system, so plus one for that!
Mac: I don't mess with Macs at all (unless my friends break them and I have to rescue them). I think they are overprices for the hardware included in them. The slogan that it just works is bogus. My room mate has his research on a Mac and everytime I talk to him about it, he said he needed to rebuild some library. He is a Linux user as well, but he has a mac for research now. The OS isn't free, neither is Windows though so no loss. One of these days, somebody will make a nasty virus for Mac. (Point, Click, queries your root password for install, and then you get pwnd). Mac just makes you use a restricted account to begin with rather than an admin account like you can in windows. I will say that macs have fantastic multimedia output though, but iTunes can suck it!
Long story short is that it comes down to preferences and usability for your needs. Every OS can get a virus, Windows was just more prominent among the common and had security flaws in the kernel so they were hackable choice. Let's break this down into business types normally associated with OS type:
Business (Banks, Stocks, Retail): Most likely Windows
Multimedia(Movies, Music, Graphic Design): Most likely Mac
Scientific(Supercomputing, Research, ultimate geeks): Most likely Linux with some Mac and Windows users
EDIT: Kevin +1 for mention of the shells. Windows finally has an okay shell (powershell 2.0), where Mac and Linux shells are simply amazing.
I think LUg married Linux Mint. That's all I ever see [s]he post about.