Hi
No you don't have to uninstall
Just right click on your avast icon in the system tray.
Click on updating
Click on program update.
After update down will ask for reboot.
I have yet to have any trouble doing it this way.
I have had no trouble with reg key. ( if you completely uninstall and reinstall, then you will have to re input your reg key.
I use Avast pro
Cheers