If you want to run a Linux VM on Windows 7 64bit, don’t even bother trying to get it to work under Microsoft Virtual PC. Not only is not supported, it just doesn’t work. I spent several hours trying to tweak parameters to no avail. It looks like it is trying to do the virtualization at a lower level which would give better performance with the trade off being compatibility issues. It does work great for running Windows VM’s, though. I’ve used the Windows XP VM, and it works fine.
In order to get my Linux VM running, I ended up using VirtualBox for the virtualization software. It was shocking how easy it was to get Ubuntu installed and running. The wizard’s in VirtualBox step you through mounting the ISO and booting up the VM. Eventually I want to get a website using Mono up and running to play with. For now, I’ll just use it for some cross-platform browser testing for my client’s websites.
If you want to run a Linux VM, try VirtualBox. If you only want Windows VM’s then either Virtual PC or VirtualBox will work fine. On the server side, I’m still biased towards Hyper-V.