Your sentence is contradictory. Windows Defender (Assuming you're running Windows 8, 8.1, or 10), is Anti-Virus.
I mean installable anti malware. Then again Windows Defender is completely incompetent.
My pick would be the HP stream 13. It is sub $300 and has all the necessary things such as office and web browsing. https://www.microsoftstore.com/store/msusa/en_US/pdp/HP-Stream-13-c193nr-Signature-Edition-Laptop/productID.333088900