Can't you legally download a copy of windows and use the key attached to your case?
I'm not a lawyer, but as a rule of thumb I'd say such distribution (if unauthorized) is almost certainly illegal, and if you partake in distribution while downloading (which happens in almost all p2p networks such as torrents by definition), it definitely isn't legal.
So, legally you are at best in gray area or directly outside law, depending on your local laws.
Morally and ethically? Microsoft couldn't give less of a damn where you get the install media for your OS, as long as it matches the (legally acquired) lisence you have. And considering how most software corporations claim you don't buy the software but the lisence to use it, I'd guess you could just as well contant MS directly about acquiring install media and activating with the old PC case's credentials.
Or you could contact your manufacturer before deciding they won't provide the disks anyway. Who knows, there might be some companies where use of healthy common sense is not forbidden. If you explain the situation - that the restore partition has been compromised - they may or may not send you the OS installation disks. If they say you should have made a back-up re-installation disk, just tell them you did but the disk's data integrity has decayed (optical storage media can do that very plausibly).
Then, if they absolutely refuse to send you the Os installation disks, you could contact some consumer protection agency or what have you. Bad publicity would almost certainly be more harmful to the company than sending you the install media...