For the most part, you should have received a recovery disk (or instructions on creating one) when you purchased your system. If you do not have a recovery disk, you may need to purchase one from your laptop manufacturer.
Your laptop came with an OEM license. That plus the recovery disk you should have is likely the reason Microsoft disallows OS downloads when using an OEM license key.
The good news is that ordering a recovery disk is often much cheaper than an actual new license. The bad news is that it may take some time for it to be delivered (unless they allow you do download it from their web site).
-Wolf sends