The first place where the virtual hard disk wizard appears is when you are creating a new virtual machine.  If you select to Create a virtual hard disk using advanced options in the new virtual machine wizard you will seamlessly transition into the new virtual hard disk wizard..The second place where the virtual hard disk wizard appears is from the virtual hard disk settings page, if you click on the create button.

Now, you may be wondering what to do if you want to create a virtual hard disk that is not associated with a virtual machine?  The answer is that you do not use Windows Virtual PC!  With Windows 7 virtual hard disk management is now part of the core operating system.  You can create virtual hard disks using either Disk Management or DiskPart.

Source (with screenshots): http://blogs.msdn.com/virtual_pc_guy/archive/2009/06/26/creating-virtual-hard-disks-with-windows-virtual-pc.aspx