Aperture keeps prompting for my serial number...

I was cleaning some stuff off my hard drive the other day, and ever since then, every time I open Aperture, a "Licensing" window comes up asking for my name, organization, and serial number.

I enter the serial number, and everything works (and my library and all of my pics are still there). But if I close Aperture, the Licensing window pops up the next time I open Aperture.

Can anyone tell me what's going on? I was pretty sure everything I was deleting from my hard drives was fine to delete, but the timing can't be a coincidence.

How can I fix this?

Thanks