You can convert a physical disc to .iso with a free program like Daemon Tools lite. There are other programs out there (magiciso, poweriso, imgburn, but I’m not familiar with those).
So just pop your disk in your drive and use that tool, and it’ll let you create an iso to use for your virtual machine.
If you’re a member of MSDN (Microsoft developer’s network), you can download ISOs of certain OSes, depending on what’s setup to you (Through my school I can get 7, Vista and XP).
And most Linux distros are available as ISOs on their websites.