You can use the Resend avast License function
http://www.avast.com/resend-license-paid.php, you will need to enter the email that you used when purchasing the license, e.g. the one that the original email with the link was sent.
From then I would suggest a clean install of avast Pro:
- Download the latest version of avast, 7.0.1473
http://files.avast.com/iavs5x/avast_pro_antivirus_setup.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear.exe
find it here and save it to your HDD (it has uninstall tools for 5.x, 6.0.x and 7.0.x).
- 1. Now uninstall avast! (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 6.x if previously installed and then for 7.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.
Note: The default install location for avast is C:\Program Files\AVAST Software\Avast.
Programs & Features, Uninstall/Change
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The above is based on avast Pro, so I hope that is the version you are suing and not avast! Internet Security ?