You NEVER want to use Windows Defender as your Antivirus. It falls WAY below any measurable standards for Antivirus, and doesn't stop Malware. You can get plenty of FREE Antivirus options out there, like AVG, Avast, Avira, Panda etc. but also need to install a AntiMalware Scan program like Malwarebytes that you will have to remember to MANUALLY run once a week (unless you buy the product then it has a automatic feature).