First, restart the computer but uncheck the box "Reopen windows when logging back in" prior to shutting it down.
The next questions relate to when the message appears:
- If it happens when you start the computer using the power button - read and follow these instructions: Starting the computer in "safe mode"
Be sure to read the whole document before doing anything. Booting in "safe mode" will not solve the issue itself but it will allow you to take additional actions to solve it. Post back here after you do this.
- If it happens when you launch Safari, launch Safari while holding a shift key. This will cause Safari to launch with a blank page.
First - open Safari Preferences and make sure that the 'bitdefender' website is not selected as home page, nor is listed in "New windows opens with" or "New tabs open with". Also make sure that you remove all references to the culprit in System Preferences/Users/Login Items.