virtualbox.org

Hi, my question is this. Can you use your own computer as a client if you're running a virtual server in VirtualBox or do you have to install another virtual machine as a client to do this?
Reason I'm asking is so that i can show my students how managing GPO's in Windows Server will effect clients.

You could use a tool by Microsoft called disk2vhd, which makes a VHD file out of your system. You can use this file as a hard drive in your VM.

But, if this is a Windows Server and (most probably) a domain server, I foresee problems if you have two of them running at the same time; 1) your physical server and 2) your virtual server.

If you want to try this for a tutorial, I would suggest installing a new domain server in a VM, with a few clients in VMs as well, that are isolated (internal network) and do not affect your physical setup.

Are you wanting to set up your guest as the domain server, and then join your host to the domain being controlled by the guest server? Might run into a couple of issues with that. First, joining a PC to a windows domain requires a reboot of the PC (at least it does on all the XP clients I handle at the office). But to reboot the PC hosting a server guest, one would have to shut down the guest first. Then one would have to fully reboot the host before one can get the guest running. Can a full domain join take place while the server is shut down? I don't know, I've never tried it.
Also, it's recommended not to have a host be part of a domain a guest is controlling because under certain circumstances the host OS needs to update things with the domain controller, which isn't running yet, which would leave one locked out of the host. The Windows 7 host I run our office SBS2003 domain controller guest on is not a domain client.
Best bet would be to run another guest for this demonstration.
Do keep in mind Socratis' suggestion to use an Internal Network between the various guests, or bad things will happen...