my program has been trying to update for some while and it cannot.I don’t know what to do.It says the license key for this product has expire or is invalid.your system time/date is not set correctly .fix it and enter license key again.your license has expired .register again and enter new license key.My question is HOW? I will be glad to do it so avast can update but how do I do it.HELP.I am not that knowledgable about these things and this is drivingme crazy.Should I uninstall avast and install it again?
Check if your computer clock is showing correct date/time. if not, set it to valid (today’s) date. It the clock shows wrong time after some time again, change CMOS battery. If time/date is correct, register your copy of avast in website to recieve new key. 
How long have you been using avast 4 Home and did you previously register?
Right click on the avast ‘a’ icon and select about avast that will show you the expiry date of your updates, if that is OK, next step.
Check that the system date (month and year) is correct, double click the clock to check, if OK, next step.
Use the avast! 4 Home Registration page and fill in your details and a new key will be sent to you.