A few years ago, there was an operating system called Lindows, that had the graphical UI of Windows and the Unix back-end. It was sold for a short while on PCs sold at Walmart. However, I remember them having trademark issues with Microsoft due to the similar name to Microsoft Windows. I believe they became
Linspire - The World's Easiest Desktop Linux
Apple's operating system, OS X, is also Unix based.
Apple - Mac OS X - UNIX
There are many free Unix distributions so generally it's just downloaded from the web, burned to CDs, and then installed on a blank disk. We use Redhat Enterprise (not free).
Unix based operating systems are designed to be low-overhead, fast-response, and robust at handling system errors. Unix generally requires less overhead in the way of CPU and RAM. You also have the benefits of open source, so you're going to pretty much be able to find any application that fulfills your needs for
free. It's worth mentioning that Redhat offers a number of Enterprise solutions,
redhat.com | Enterprise Linux, which are not free.
Windows has become more reliable as of late, but it is still prone to crashes that require server reboots. Because the core operating system of Windows Server is near that of Windows XP, you also have a lot of the same security vulnerabilities that need to be patched. Windows Update makes this easier, but you also run the risk of auto-installing patches that require server restarts and/or cause issues with existing software. The fact that the computer is Windows based also means that it's going to be a more frequent target to worms and viruses. The per-seat licensing fees on Windows Server also makes it less desirable when faced with free alternatives like
The FreeBSD Project.
Both systems get the job done. It just comes down to user preference.