This proves to be more complicated than you might think. I assumed you could copy a vm (its folder with config file and settings, and its HD vdi file) to a new machine and just run it. No no no, that would be too easy, at least in the case of Windows vms. What you have to do is to clone the vm on the original machine, then copy the clone to a new machine (make sure you never run it on the original machine before moving it). Then when you first run it on the new machine, you'll have to re-activate it (choose the least intrusive option when asked; it works OK), and then it'll run OK.

Other things I've learned in the process of migrating machines from one Linux host to another: my new machine did not have Intel VTx virtualization options turned on in the bios, so I had to do that. I also had to reconfigure a couple of my non-Windows vms after moving them, because they were originally set up as generic "Linux", but were actually CentOS 64-bit, and on the new machine they wouldn't run unless configured in the vm settings as "Red Hat 64-bit".

I haven't yet moved the really crucial Windows 7 vm. I'll report back after trying that.