First confirm that the system date day/month/year is correct 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.
If that doesn’t resolve the problem a clean reinstall may be required.
I don’t work for avast, I’m an avast user and have basically laid out the common causes of this and the steps (in least hassle order) to try and resolve it.