I'm afraid the only option is to uninstall, reboot, install again.
It would probably be best to download the latest version of avast
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download this before you uninstall so you still have protection, use this when you reinstall.
If you still have the registration email you were sent with your registration key, find it in readiness.
If you cant find it, complete the
avast! 4 Home Registration form and you will receive an email from register @ avast.com (without the spaces) with your registration key. Again if you need to do this get it before you uninstall, so that you have everything you need before you uninstall.