To what extent is this legal?

The trick of it is that with Windows, you aren't buying the disk. You're buying the software on the disk. Just because you lost the disk, it doesn't mean you've lost rights to the software. Microsoft could care less if you torrent it- they know a case against it will get thrown out. My best advice would be shooting Microsoft tech support an e-mail explaining what happened and asking if you can get a new disk that way, including your license key. If they say no, it may violate the EULA. It's up to you how far you want to go from there.

Downloading Windows is not illegal. What's illegal is installing it without having purchased a license key.

Honestly I don't like using Torrents (nor do I support it) for anything but since you aren't obtaining anything that you don't already have then I would say Microsoft probably doesn't want you to do it but is it illegal not really. Since you are using your own key you should be okay. The better way to avoid doing this is to just ask a friend and barrow theirs that way you don't worry about what you are downloading.

I don't see anything wrong wit doing that as long as you have not used the key on another computer already.

when you purchase anything with a serial key you are purchaing the key to activate it downloading the appropriate software is fine along as you use a bought serial key ;P

There is nothing wrong in downloading the OS itself, It al comes down to licensing at the end of the day. Since you have a legit key, you should not have any problems?

through previous experience, not all keys are compatible with all disks. you may have to download several disks to key a key that will work.

you can download copies of windows with no problem, you just need a valid serial as that is what they are actually charging you for,

They have no problem with people downloading their OS, which is why you see many sites offering pre updated copies. Just make sure you are getting the download from a trusted source.

Its completely legal, when you purchase windows you are buying the right for you product key, you can obtain the actual OS by any means as long as you use the product key you "own"

You still have the rights to the software so yes you can torrent it for personal use as long as you paid for it in the past for your own use.