1) Thoroughly uninstall any Antivirus from its official uninstall tool from here
Uninstall Tools Links For Antivirus Softwares
2) Perform the procedure for Novice or Advanced user according to your skill level to remove all the remnants and traces of previously installed Security Software from here
How To Completely Uninstall Bitdefender, Detailed Procedure
3) Disconnect your system from internet and then try to install Bitdefender with offline installer, just ignore when it asks to check for the newest version/updates. After successful installation, you can enable the internet connection and update your product.
The installer type (32 or 64 bits) should be according to the Windows (32 or 64 bits) installed.....means 32 bit installer for 32 bit Windows and so on.
Bitdefender Offline Installation kits
Bitdefender 2011 Version
Bitdefender 2012 Version
Bitdefender 2013 Version
I'll appreciate if anyone suggest further precious instructions and tools regarding the subject.