Microsoft is the only company with an actual .lit reader; it's their proprietary format. (For all I know, there's a way to read them in Internet Explorer.) Calibre converts .lit to other ebook formats very well (.lit is its preferred source format; converts well to pretty much everything), and Convertlit and the GUI version are both free, simple programs that convert .lit files to .html files.

Convertlit will run portably on a flash drive. (So will Calibre, but it's a lot messier.)