DavidR
2
What version of avast are you using, Home or Pro ?
I think the message date could be a little scr*wed (sorry about getting technical) as the date January 01 1970 is wrong, I doubt avast were issuing keys in 1969 (license is one year for pro, 14 months for home).
Or if pro, where did you get your registration key as a valid till 1-Jan-1970 seems somewhat strange. We have had this before for I believe the same date when the license key came from a keygen and not avast. Periodically theses keys are detected and the VPS updated to block them.
If Home version, just re-register, complete the avast! 4 Home Registration form and you will receive an email from register @ avast.com (without the spaces) with your registration key.