Short paths:
(server) FreeBSD -> FreeBSD and some Linuxes -> FreeBSD
(desktop) DOS -> DOS + Windows 3.11 -> Windows 95 -> NT4 -> NT4 and Mac OS 7.5.3 -> NT4 and Mac OS 8 -> Windows 2000, Mac OS -> Windows 2003 -> Windows Vista + FreeBSD in VMware -> Windows 7 + FreeBSD in VMware
Long answer:
In 2001, we made the project to buy a dedicated server on my IRC channel.
We picked FreeBSD as operating system for a web/irc/mail server.
I don't remember the crucial factor but here some points leading to our decision:
(1) One of our users wanted to install gnuworld (it's the channel management service on Undernet, X) and at this time the howto were optimized for FreeBSD.
(2) Our sysadmin had a preference for FreeBSD (but I think we found it after the FreeBSD choice).
(3) This channel were a Windows support channel and we were tired of linux kiddies flooding our channel saying linux rulez, Bill gates is evil, Windows go to hell.
(4) FreeBSD had an excellent reputation on the Undernet IRC network, more than an half of the servers used on FreeBSD.
(5) They were some echo than FreeBSD were more stable than Linux.
(6) We were young males, teenagers or young adults. Beastie seemed more cool, virile and powerful than a pinguin.
Some months later, we needed more capacity and took a second server on Debian.
Meanwhile, our sysadmin resigned and if we had another sysadmin for the Debian box, I were the main sysadmin on the FreeBSD one. I so were able to compare Linux and FreeBSD and prefered a lot the FreeBSD one.
Probably the sockstat killer command and the port system.
Nowadays, I always use FreeBSD as server for 10 years.
For 4 years, I use also it as desktop coenvironment: my laptops are Windows + FreeBSD inside VMware. I'm happy to be able to get the advantages of the two worlds (ability to run demos - as demoscene releases - and apps like Photoshop, Excel or Visual Studio on Windows, to have a nice cli and Gnome to edit my web projects with a dev environment similar to the prod one FreeBSD side).