1. without entering the license key, your trial/registration period would end and the very least you would be unable to get signature updates, at worst the program wouldn't function.
2. you can't use the old license as that registration period, which is incorporated into it would have expired.
You seem to be living in the past:
hi,i registered avast !4 home edition
last mon, apr 20, 2009 o5:40pm and i received confirmation that it was successful. i forgot to insert the license key.but the icon at bottom of screen were working.now it's is pass due for reregistering.
Since last Monday was 24 May 2010, yes it would be well past re-registering time.
There is also a new version of avast, 5.0.545 and I would advise installing that (not if you have win9.x or winME) and registration is a much easier process, directly through the avastUI (User Interface).
Edit: I have just noticed the effective duplication of this topic,
http://forum.avast.com/index.php?topic=60199.0.
Please don't post multiple topics for the same/related question, it just duplicates the effort for those trying to help.