You know what? The vendor knows very well that you’re going to loose the product key. Why do you think they give you that cheap little paper CD envelope? I know why, because they want you to run out and buy another copy.
This is what I do
I create a copy of the software before I even install it. One reason is for safe keeping – just in case the CD gets damaged or lost, and the other reason is I embed the Product key into my copied version so I never have to worry about loosing it or having to type it in ever again!