@David-Kaye Is the software DVD Architect Studio 7.0? If it is, did you install it on a second machine? If you did, you need to deactivate it on that machine. DVDAS 7.0 can only be active on two PCs. Go on the Magix website and log in to your account. Select My Products and find the DVDAS that you registered. If it was DVDAS 5.0, I think that you can only have it registered on one PC so you need to see if the serial you're trying to activate it with is the correct one. If it's DVDAS 7.0 you can deactivate the software that was on the old machine and then you should be able to activate it on your new machine.
Unless, of course, you acquired your old machine with the software already on it. If this is the case, there's not much you can do. If you have the serial number you could contact Magix and see if something can be arranged.
EDIT. Ah, @set. You squeezed in whilst I was typing.😉