I still have XP on my main machine... because... well it works well... but then again, I have a crap load of machines (just about every OS known to man, lol). I run every operating system from WinXP-Win7 (I have a machine with Win8 but haven't played much with it), , Win Server 2003-2012, CentOS, RedHat Enterprise, and Ubuntu (various versions).
I just got a new one that will soon takeover as my main machine, it's got a Core i7 with 32GB of RAM and two NVIDIA video cards, it's going to be multi-boot with Windows7, CentOS6, and probably Linux Mint w/ Cinnamon(the latter is the only one not installed yet but I've had it with Unity).
And both our desktops are over 12 yrs old. A dual PIII and a dual Athlon MP. I'm running a RAID card in the Athlon where the management software work with any drivers newer than XP because they don't have the proper signatures.
Keep looking at newer ones, but there just isn't money in the budget for new ones now that we have 2 kids in school.
Just upgraded from XP to Win 7 at work last fall when they finally upgraded some of our 5+ yr old computers.