Indeed, I have a sneaking suspicion that IE5 means Quirks mode, because it's probably more understandable that way for the less savvy web developers out there (i.e. the ones who're still writing pages in/for quirks mode)
IE6 was the first version of IE to distinguish standards and quirks mode with the doctype switch. Quirks mode essentially meant "render using IE5's incorrect box model".
IE7 still only had standards vs. quirks mode. It couldn't emulate IE6's standards mode.
IE8 has quirks, IE7 standards, and IE8 standards modes. While it could emulate IE7, apparently they didn't feel the need to emulate IE6 standards mode as well.
IE9 just adds its own new IE9 standards mode, and the ability to emulate IE8 standards mode. It still can't emulate IE6.
As Andy speculated, I agree that "IE5" in the preview simply means quirks mode. IE6 is missing because none of the earlier versions could ever emulate IE6 standards mode.
EDIT: One of the reasons why the real IE's installation is comparatively slow is because most of IE's files are critical OS components that are under Windows File Protection. Replacing those files takes a little more doing than just overwriting them (if
you did, WFP would simply put the old version back). The preview doesn't replace anything (since it's installed side-by-side) so it doesn't need to do that and can install much quicker.