Check out the "downloads" link in the left margin. By installing the Text-to-Speech engine it will add 2 voices. For english that would be LH Michael and LH Michelle. But if I'm not mistaken, those two voices also come by installing OfficeXP Speech components. SAM on the other hand is provided by the OS as part of the "Accessibility Options > Narator" in WindowsXP. I can't select other voices for Narator, btw, even though I do have LH Michael and LH Michelle show up in the Speech Control Panel application.
However, you may want to take a look at the Microsoft Agent software, too. You have more control over these so called "agents." They are VERY easily controlled thru HTML script. Microsoft gives you tons of examples, but these guys
You will not, however, find the voices of the agent characters in the control panel speech. You can find lots of free or commercial agents on the web. Some of the freeware ones are really cool! A good place to start would be here: