debian fedora opensuse etc and vm

So in virtual box can you toggle between 2 or 3 running boxes at the same time?

If say at present all you have is windows 7 is that the host OS? Do you install virtual box on top of windows 7?

How would you go about creating a new ubuntu box within virtual box? Is it an easy process?

Can you run more than one system concurrently or when you go to the new system does the other shut down or something/

we can toggle between 2 or 3 running boxes at the same time.
we install virtual box on the top of windows 7.
       this is the same case if we install vmware workstation, but if we go for vmware esxi or sth like that called we install hybervisor on the system and all other system runs over it after that.
creating vm is easy in virtual box, we need guest os iso or cd mounted on virutal cd. it will be more clear when you create a virtual machine it will guide you to assign memory (RAM) size, hard disk etc.
we can run more than one system concurrently, parallely more than one virutal machine, only the bottle neck is the size of RAM in your system (real system). once you load too many virtual machines your system performance will degrade. but you can allocate or reduce the RAM size for the virtual machines to run more vms. just say 128MB ram or 256 MB ram. I am running 2 vms with 256MB ram and 1 128MB ram vm in production in a system of 1GB ram.

So in virtual box can you toggle between 2 or 3 running boxes at the same time?

Yes, as many, as you have resources for in the host (Windows 7).

If say at present all you have is windows 7 is that the host OS? Do you install virtual box on top of windows 7?

Yes, Windows 7 is the Host OS. Virtualbox is an application, you install it in Windows 7.

How would you go about creating a new ubuntu box within virtual box? Is it an easy process?

Yes, you Create a VM and follow the wizard.

Can you run more than one system concurrently or when you go to the new system does the other shut down or something/

See first question, in this post, you can run many concurrently, depends on resources in the Host. CPU and Memory, Storage.

Ok thanks. I downloaded a ubuntu image from:

http://virtualboxes.org/images/

I followed the create new disk image in virtual box, and it worked fine, however I never got any choice in creating the root/admin user password 9for the ubuntu virtual machine, so I've no idea what it is ! Should that be?

Yes, thats it, if you.ve downloaded an image nothing more to do.

instead download the iso image of ubuntu and then create a new virtual machine and follow the wizard, you can set up root username and password as your will