I recall this being asked before, possibly even answered. Oh well.
For the most part, older versions have been lost forever. Until GameHippo was abandoned, you could get an early beta of the game as well as the latest version they knew about, but it's long gone.
... So at this point, the oldest version you can reliably expect to find is the last Notrium release before Cormoon development kicked in, which IIRC is v1.341. If you've got Windows Vista/7, though, you're probably best off running the latest version anyhow because it resolves some important issues.