I honestly don't see how this can be directly connected as the virus definitions update doesn't change the programs functionality and avast shouldn't/doesn't have any ability to change keyboard layout/use built in. I'm not saying this isn't happening to you, just that there has to be something else in the mix.
What other security software do you have installed (firewall, anti-spyware, anti-keylogger, threatfire, etc.) ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?