There certainly seems to be something else wrong as I have said even if the registration had expired (and from what you say it shouldn't have), the program should continue to work. The only exception being you wouldn't be able to get VPS updates.
Is the avast ' a ' icon in the system tray ?
If so right click it and select about avast that gives build information and expiry dates, etc. see image.
First try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.
You could re-register again and use a different email address, that should give a different registration key and see if that one works.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
I suggest you remove your email address unless you particularly like spam, email addresses in publicly available web pages are likely to be harvested by a bot.