that the original monitor was manufactured three years prior to your purchase is not necessarily proof that there was anything untoward. Three years is a very long time for the manufacture to retail cycle however, and I think I might be uneasy if a monitor broke down in the same circumstances.
Having said that, it isn't at all uncommon for a manufacturer to replace a faulty item with a refurbished used item - it happens a lot. The fact that the original monitor failed within a very short space of time is grounds for you to contact Daewoo again, and tell them that you assume (as does the law) that the fault existed at the time of purchase, and that the monitor was not fit for its purpose. Say that you are therefore rejecting the monitor (even though they've sent you a second-hand replacement), and will accept a new model, or a full refund of your money.
Try that - keep a note of all relevant calls and names, and come back to this thread to tell us what happens.