All but one of my computers are running Windows 7 Home Premium SP1-including my main laptop, my spare desktop (which I upgraded from XP), and another desktop which I set up primarily for my mom to use and learn computer skills on. My oldest machine, however, runs Windows 95 OSR2, which I occasionally run just for the nostalgia. My laptop also has a Lubuntu (a lighter variant of Ubuntu, which I had previously used on my spare desktop in dual-boot with XP, before upgrading to Win7) virtual machine in VirtualBox, as well as VMs for Windows 2000, XP, and NT 4.0, the 2000 and XP ones I use primarily for running older applications not supported by x64 Windows 7.
If it weren't for some of the proprietary (Windows-only) software packages and devices that I have, I'd probably have an all-Linux fleet.