Is it okay to download a copy of old software which you already own

I assume you are asking if it is ok because you are operating outside of Microsoft's standard channels. If you purchase a copy of a copyrighted work and lose your copy, your purchase does not give you license to make another copy to replace your lost copy. When you had the purchased copy, you would have been permitted to make a back up and restore or transfer the back-up, but when you lose possession that ends your licensed opportunities. You can try to explain your issue to a Microsoft vendor and see if they will give you a copy for free, but I would not expect that to be a fruitful exercise. Good luck, Todd