It's always advised or more of a rule to uninstall current antivirus programs before you install something else from the same category. They might work together very well, but conflicts will appear at some time making it impossible to clean the PC effectively, therefore leave only one installed to have maximum efficiency.
And yes, that could be the cause to the issues you've encountered. Remove the one installed, then start the main installation for BT NetProtect Plus again and it should work.
If you're using a modern OS, right click the installation setup and select Run as Administrator.