Hei guys,
I've got it working.
I have tried to register it through the Avast's user interface several times before. I have had the same problem as well: When I press the 'Register for free licence' button, nothing happens! The button is inactive or "greyed out". I am on this forum because my trial was ending and I was looking for resolution.
But I've tried it once again today and it worked.
I did it with a built-in administrator user (not another user with administrator's privileges, the way I had tried before), and my Windows firewall was off (though I am sure it was off every time I had tried before).
I am running only windows XP sp 3 installed from scratch, there is no ANY other security software installed (in fact there is almost no other software at all, except the VLC player, Adobe flash player 10 Activex and the Quick time).
Apparently the issue was not due to a software compatibility problems.
Hope it helps.
Cheers