OK…

Two options:

  1. Uninstall avast ( you can use ‘avast! uninstall utility’
    http://www.avast.com/i_idt_1336.html) and install again. Use your key to register.
  2. Install avast … over existing installation. I chose this method, and I can use my key…

After all, even if everything looks ok now, I’d like to know why and what happened to my registration key…
Where is the key written down and keeping?

Is there any body to answer my questions?