You can probably get replacement media from the laptop manufacturer. I don't think you can download it from Microsoft. Or you could find a friend with install disks and borrow them. This is not breaking any rules since you will activate it with the key from the laptop. OTOH you cannot use that key to install the OS on another computer, that would be breaking the rules. OEM licenses are not transferrable.
If you are trying to reinstall on the laptop, do you have access to the original hard drive? It might have an image of the OS installed on a hidden partition.
You can download Windows7 and install it with your laptop key, however it will not activate as the number is tied to the laptop hardware. Depending on the type of licence the laptop has if you phone Microsoft they may (if they are feeling generous) give you another key. It is worth a try. Report back here and let me know what Microsoft says.
Legaly, that key is tied to that laptop and you can't use it with any other computer. Any key that came with a pre-installed OS, especially from a large vendor would be an OEM copy which is not transferable to another computer even if the original computer is dead or not used. That is why the OEM licenses are much cheaper than the retail versions. And are even cheaper for large companies to buy than what a retail OEM license would be. Aside from the "no support from MS" thing which is pretty much useless unless you get to 3rd level engineer support level.
There is no problem with replacing a hard drive on a laptop as regards activation. You are allowed six hardware changes to counted devices before the activation fails. Unfortunately changing the motherboard is counted as more than six changes
I dont know where you got that information from ? because a Laptop cannot purchase a key nor is a laptop the registeree.
Secondly, Are you trying to tell me that if your pc was mortally damaged, you are basically S.O.L
Umm I dont think so.
I have done it a million times....as "UNKNOWN DEVICE " stated and he is totally correct ... IT MUST BE THE EXACT VERSION THAT WAS REGISTERED AND ACTIVATED / PER PC to include x32/x64 (which is generally the activation denial !!!)