Virtual PC versus VirtualBox

VirtualBox is constantly being improved. I've seen it get a ton better in a short time. It has great features over what VirtualPC can do. The USB support, 3D Graphics support and Seamless mode are great. I use it on Linux, Windows and Mac hosts for Linux and Windows guest OS's.

VirtualBox is my preference; I can run my VMs from other OS's. Virtual PC locks you into a Windows host.

VMWare Workstation is probably the most feature complete, but you'll have to pay. If you decide to switch from using their Windows version to their Linux version then you have to pay again.

VirtualBox has a solid feature set, is free, and runs on a variety of OS's and virtualizes a variety of OS's. Their feature-list is very competitive with VMWare.

VirtualPC hasn't offered much I like. You can use it on Windows to run Windows. Great, but Windows as a Host OS is a fairly poor choice performance wise. Personally I think it's just to keep Microsoft from telling customers they have to use another big-name product (like VMWare, look at their stock prices if you don't think they're large).