If
@buddrousa's copy and paste solution does not work, try right clicking the shortcut for Win96 and run it as an Administrator.
I read a similar post from someone who could not get Win500 to register on their Windows 10 machine. Running it as Admin allowed the registration key to work. Not sure if this will help but it's worth a try if you are using Win 10 or even Vista or a Win 8.x version.
I've used my original keys for both Win500 and Win96 many many times and have never had an issue with them not working.
I always used copy and paste to eliminate errors like
@buddrousa suggested above. I always keep copies of the original email for every program I've ever registered as well. You just never know when you may need to info again!