Avast 4.0 license/key will not work with v.6.0x, and as already mentioned, v.4.0 support is being discontinued any day now. You do not mention what version you have installed, but you should have the current version 6.0.1000. There is also a Pre-Release v.6.0.1044 available.

  1. Save a copy of Avast and save it to your HDD:
    http://www.avast.com/download-software – all products (Free, Pro, AIS)
  2. Download the new Avast Uninstall Utility http://files.avast.com/files/eng/aswclear6.exe and save it to your HDD.
  3. Disconnect from the Internet at this time.
  4. Go to Control Panel and uninstall Avast through Add/Remove Programs if possible and reboot.
  5. If Step 4 fails, boot into Safe Mode (hit F8 repeatedly) and run the Avast Uninstall Tool. Uninstall ALL previous Avast versions and products on this machine at this time.
  6. Reboot.
  7. Install the newest version of Avast and reboot.
  8. Get Internet access and register your copy for Free:
    Free – http://www.avast.com/registration-free-antivirus.php
  9. Update the Avast definitions.

Knowledge based article:
https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=504#idt_0003

Make sure you have the most up to date version of Avast installed and try registering this via the GUI (not by email). If you have v.4.0, I would do the uninstall/clean install. See if this fixes your problem and let us know how this works. Thank you.