Though if the old hard drive is IDE, it might not be usable with the new motheboard unless it has an IDE port - which are becoming less and less common on new motherboards (and have been for a couple years at least).
However, it's very likely you'll need to reinstall your OS; I've put together six or so builds for people, and only once was I able to use the old OS without any problems (and even then, it was a great surprise). This is because the old OS still has all the old drivers and such, and it has a hardware profile of all the old hardware. As soon as that hardware changes (depends on how different this new computer is), it'll look at the new hardware and go "wuh, what is this?!"