what version of Avast free are you using ?
I use avast! Free Antivirus 5.0.396
so don't bother at all with the online registration page, just fill up the form in the UI to registrer, and you'll be instantly registered. No need to back up any key (I don't even think there is one, but just an entry in the registry or a file to keep the record, can't check now, I'm on AIS), this is just done for statistic purposes in the free version and you'll be able to register in the same way again and again (in case you'd have to reinstall). I've never used this online registration page when I was running the free, but they might send a code similar to V4, not sure...don't bother with that

(One thing is sure, you can't use a V4 code, but there's also a way, a button, to enter such a code in V5 free, as opposed to pointing to a license file like for "pro" or "IS".)