I don't know about pre-installed trojans, but I wonder why people would pay 150 (+license-software) for what they could get free. You trim linux once in to the shape and it's good for years (albeit updating more often is a good idea

). With M$ you can hardly go three days without having to update. And funny thing is that on default the system tells you what to do, not the other way around.
windows is the pre-installed (possible) trojan.
i also update often, there are security holes in linux too.
but with windows you
need to trust microsoft, you don't have any clue about what happens on your computer, because windows is closed source.