MSE is an app available for free from MS. It is a AV/AM app which runs in real time. If you want to try it, disable the present AV app (Bitdefender) or better yet uninstall it completely using Revo Uninstaller, then do a Google search for Microsoft Security Essentials and download and install it.
edit: download MSE first, then disconnect from the internet to uninstall Bitdefender, then click on the MSE executable file to install it. This will preclude any nasties getting through while you are installing the new AV. Most experts agree that only one AV/AM should be run in real time as running 2 or more can cause conflicts.