From your issue description I understand that after upgrading to Windows 8, BitDefender and Windows Defender is not working. Do not worry; I am here to help you.
Windows Defender and BitDefender are both antivirus and antispyware. It is not recommended to have two antiviruses running at the same time as they can cause conflict which is what the OP is experiencing.
Windows Defender is included with Windows and runs automatically when it's turned on.
You can disable third party security software, bit defender installed on the computer and turn on Windows defender. Otherwise there will be a conflict with the security software.
Disclaimer for disabling security software: Antivirus software can help protect your computer against viruses and other security threats. In most cases, you should not disable your antivirus software. If you have to temporarily
disable it to install other software, you should re-enable it as soon as you are done. If you are connected to the Internet or a network while your antivirus software is disabled, your computer is vulnerable to attacks.
If you choose Windows Defender since it is built into the operating system, then you have to uninstall BitDefender. If you choose BitDefender, you need to disable Windows Defender
Refer this link for more information on Windows 8 Defender.