You shouldn't need the 2005 version for a 64-bit. Try a cold boot to your machine. However if you find that your PC date/time is wrong on boot up, restart it in Safe Mode and correct the date/time, then restart the machine in normal mode and see if Avast will update itself on it's own. If not try a manual update under Maintanance.
What version of Avast are you using (Free, Pro, AIS)?
What other security software do you have on your machine?
Under the Avast GUI > Settings > Updates ...how do you have this set and how do you connect to the Internet?