FFDShow shouldn't ask you for a certificate at all.
Or did you mean Opus is asking for a certificate? If so then see the FAQ: Registration Certificate suddenly expires or does not stick.
You do not have to pay for minor Opus updates so if you've updated from version 9.X to version 9.Y you should not have to get a new certificate. Only major upgrades require new certificates, e.g. version 8 to version 9.
(One possibility not mentioned in the FAQ is that you're using a pirate certificate. New versions tend to block those...)
Also: You still haven't answered my question about Windows Media Player. Do the videos play in it or not?