OP, Windows Defender is probably your best bet if you're determined not to pay anything. However whilst Windows Defender is more than adequate, there are better performing AVs out there according to av-test.org and av-comparatives.org, the 2 best AV comparison test websites.
NOD32 (ESET), Kaspersky and Norton/Symantec* are the 3 best consistently performing AV suites. But the trick is NOT to pay the official inflated prices, for example Norton charges around £80 pa for a 3 pc licence lol. You can get genuine licences on Amazon or eBay for a tiny fraction of the RRP.
* Norton is no longer the bloatware it once was (c. 2005), its now amongst the top performing AVs out there