So I was working on a family member's laptop. The Windows product key on the bottom had faded really bad to be almost unreadable. Some laptops have the product key under the battery where it won't get rubbed and faded so much, but most don't. The serial number was also pretty much gone although I have seen one netbook that actually protected the s/n with a clear coating. Another annoyance of product key activation having to worry about losing the key and thus having higher costs to fix something.
Short lesson is to make a backup of the entire bottom of your laptop to make sure you have everything even just 2 years down the road otherwise you may be stuck buying your OS again. I just make a scanned photocopy of the entire bottom.