Removing any software from your Windows computer is a simple task that one can do by going to Add/Remove option in control panel for Windows XP or Program and Features in Vista & Windows 7. I did the same way but still the ESET online installer showed me the message for the error. I already posted about the Norton Removal Tool that I used to remove the Norton software and while the computer was restarting I searched for the Avast removal tool instead of manually deleting the registry keys.
You can download the Avast Removal Tool from the developer using the below link
Once downloaded, just run the tool in safe mode as recommended by the tool, select your installed product and click uninstall button as shown below
Incase you use the tool in normal mode like me, you will see the self protection module enabled message as shown in the image
Now let us see how to disable the self protection module. Simply open your installed Avast software and click on settings option on the top right
Now just go to the last option which is troubleshooting >> uncheck the enable Avast self defense module option and click on ok
You will see the self defense turning off warning, in this case you should click on yes otherwise if you have not changed the option, you should always click on no.
Now once again run the removal tool, select your installed product and click on the uninstall button. You may see a warning message to confirm the process, click yes to proceed
Once the traces of the software will be removed, you see a confirmation message on the screen as shown below
Now restart the computer and proceed with the installation of software. After the removal I dint use the online installer and searched for the ESET offline installers to ease the installation. Incase you are using an intel based Mac, dont forget to use the Avast free Antivirus for your Macintosh. Earlier it was in beta phase but recently they have released the final version