I'm sure there are members here on PC that use Virtual Machine operating systems.. for those of you that do, what program do you use (VMWare, VirtualBox, MS Virtual PC, etc...), and what operating systems do you have in a virtual machine?
For me, I use both VirtualBox and VMWare Player. I have a Windows XP VM, a Windows 2000 VM, and a Windows 3.1 VM in VirtualBox at the moment and in VMWare Player I have a Windows 98 VM and a Windows 8.1 VM (since 8.1 wouldn't install in VirtualBox for me).
I used to be an avid user of Virtual PC, having run numerous older versions of Windows (both DOS-based and NT) as guest OSes. Nowadays, I use VirtualBox, although the only guest OS I currently have in it is Windows 2000.