Windows is the operating system of choice for business. Period.
And since January 2006, when Apple launched its first Intel-based iMac, Windows is a viable operating system for Intel-based Apple Macs (ie. all Macs sold by Apple as of this summer).
With the switch to Intel processors, the Macs can now become quite good Windows PCs, either as dedicated Windows machines (Windows is the only OS you run on the Mac) or more likely as dual-OS machines (Windows and OS X coexist on the machine, and you switch between OS depending on the task at hand).
The benefit of this setup is that you get access to very good hardware, that is built with Apple's legendary attention to details and good design. You also get access to software from both OSes, and can optimize your experience for the applications and environment you prefer - for example Windows for work (or gaming) and OS X for photos and web browsing.
There are two ways to reliably get Windows on a Mac right now:
Apple's BootCamp is a free (for now at least) utility that Apple built to allow you to split your hard drive between OS X and Windows, and to install Windows (you need to have your own Windows CD, it is not provided by Apple) in its dedicated space (a partition). You select, at boot time, which of the two OS you want to use. The two OSes are independent and exclusive of each other - only one runs in any given session (you must reboot to use the other one), and neither really knows that the other is installed on the hard drive (there are utilities to access Windows files from OS X and vice-versa).
The good here is that Windows runs "natively" on the Mac, it has access to all of your PC resources without having to go through OS X. If you want to play Windows Games on a Mac, you need to have that direct access to the hardware and BootCamp provides that (other solutions are beng developed for gaming and should be available by early 2008).
The bad is that you have to choose wich OS to use, and you cannot switch from one to the other without going through a reboot. If you need to switch between Windows Outlook and iPhoto, BootCamp is not the solution.
The other solution (the one I currently use) is Parallels Desktop for Mac, a $80 OS X application that creates a virtual PC within OS X where you can install Windows (again you need to have your own copy of Windows to do this).
Virtual PCs are not new on the Mac - Microsoft offered Virtual PC for the Power PC Macs already. That program basically translated every Windows program instruction for the Intel processor, on the fly, into a Power PC instruction. This translation overhead had a substantial impact on performance.
With Intel CPUs and chipsets inside of Macs, most of this on-the-fly translation is gone. Windows runs "almost natively" on an Intel Mac.The good here is that your non-video intensive Windows apps performance will be very close to what you are accustomed to on a Windows PC. And because you are running Windows inside of OS X (in a window or full screen) it is easier to switch from IE7 or Access to OS X to look at your pictures or browse the web. Parallels also allows you to move content between OS X and Windows - you can copy & paste from Windows to OS X and back (remember to use the CTRL key for shortcuts in Windows though), and you can access shared folders of the two OSes.
The bad here is that this remains a "virtual environment" and that you can only see and use in Windows what Parallels has provided support (drivers) for: USB 2.0 is not yet supported, graphics acceleration is not supported, etc. Parallels is working on these things, but for now you'll have to do without them.