Microsoft has changed offline installation policy and you can't download any offline installer, until you don't provide your product key and you don't login with your Microsoft account. If you don't have it or you don't want to use it, then the only option, you're left with is to download offline installer from unofficial sources.
In this case, make a use of this link or search for more in the Internet.
This is unofficial and probably illegal way, so if you have product key and Microsoft account, you're encouraged to make use of official link to obtain a copy of "lost" installation CD. Enter your product key, select language and enter captcha to continue.
You'll be redirected to Microsoft account login screen, once your product key is verified as valid.