Purchased game with the download option, downloaded the game, 4.5 hours, on entering the serial given a number of times, also uninstalling and installing a number of times get the message that my serial number is invalid, on a positive note, I am able to read the manual. Operating system XP. The download was from the Matrix sight
I'm bot 100% sure, so you might want to wait for someone to confirm this, but if you have a problem with your serial key when you have purchased the game through Matrix, then it's Matrix you need to contact to sort it out.
On a general note; are you typing the serial key? Have you tried copy'n'paste to get it into the settings-program?