Well here's the doggy's take on things.
Do you really need a program like this? Not really.
Supposedly iolo System Mechanic Pro addresses things like tcp ip settings and the like to improve throughput and cleans junk files, etc. and is considered a better solution.
Microsoft doesn't like registry cleaning utilities at all:
Microsoft support policy for customers who use registry cleaning utilities.
support.microsoft.com
Quite honestly I never got a warm and fuzzy feeling from any Avast product but that's my opinion..
If disk housekeeping is an issue you might consider doing your own inspection with WinDirStat a freeware program. Thumper is a believer in the duplicate file finder programs, he could help you out with those.
Bottom line is the Avast program isn't even a crack. Those numbers being passed around will all expire exactly 1 year after the first one from that account was activated.
Anyway I would do a little research into exactly what you might need if anything and use a resource like 1337x to find a version of that program that will work or the comments section will tell you if it doesn't.
Sorry if that's not what you wanted to hear. I would put your chances of finding someone to share their key code for Avast extremely small and even then not worth the bother.