I just downloated Win. 98 version of Avast 4 Home Edition. I got my license key e-mailed to me and put it into the program.
When I try to upgrade the virus database… It conects to the surver, downloads a few seconds and then tells me " License Key for this Product is Invalid or Expired "
Any thoughts? 
right click on the AVAST tray icon and click start avast
when the splash screen comes up click reg. and re enter the code to see if you made any mistakes
If you just download the avast and receive the email, I suggest you copy & paste the licence key number from the email to the Registry window like MacLover said 
If you still have problems, I suggest you register again to receive a new licence key.