system
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Hi…
You probably won’t be able to follow Tech’s instructions becuase users with under 20 posts are no longer allowed to PM, a change brought about since June, 2004, when this thread dates from, unless Vlk chooses to respond directly.
You will need to follow RejZoR’s instructions instead. 
Uninstall avast using this tool…
http://www.avast.com/eng/avast-uninstall-utility.html
If you’re using the Home version, reinstall avast from here…
http://www.avast.com/eng/download-avast-home.html (or whichever language version you prefer.)
If you have deleted the e-mail containing your program key, you can reapply for a new one here…
http://www.avast.com/eng/home-registration.php
and if you need help with installing the key, please see this video tutorial here…
http://www.avast.com/files/tutorials/insert_key.htm
Hope this helps. 
May God Bless you! 