VirtualBox comes with a useful command line utility called VBoxManage that allows you to do anything that can be done with the GUI tool and more. The only problem is it isn't added to the Windows path. Adding it to the path is relatively easy.
First, bring up the System properties. The easiest way to access this is through a keyboard shortcut, "Windows-Break." Alternatively, you can get there by clicking "Start," right-clicking on "Computer," and then selecting "Properties."
Now click "Advanced system settings" in the list on the left hand side.
Click the "Environment Variables..." button towards the bottom of this dialog.
This will open up an "Environment Variables" dialog. In the section labeled "System variables" scroll down to the one that says "Path" and click "Edit..."
Append the following to the end of the box labeled "Variable value" and click "OK." If you installed VirtualBox to another directory, use that instead:
Click "OK" on the "Environment Variables" and "System Properties" dialogs. Open a Windows command prompt by clicking "Start" and then typing "cmd" into the search box. If everything went well, you should be able to type the following to get a list of your virtual machines:
VBoxManage list vms