Check your system date, day/month/year and adjust as required.
If it is correct try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.
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Purchasing through a dealer may complicate this a little as I don't know how they are recorded, with a view to using the resend license option I suggested.
Are you able to contact the dealer ?