From what i have heard it's really hard to find CS5 working serial number. I would recomend download CS4 or getting the demo of the website and keep extending. I have FCP 7 so i have not looked much for AE stuff.
just get Adobe After Effects Cs5 using torrents from pirate bay, thats what I did. ALl you do is replace the host files (comes with instructions) and everything is taken care of. Obviously there are more steps but thats the jist of it. Pretty easy if you know the basics of torrents.