Microsoft include their own security software (either as Microsoft Security Essentials or Windows Defender) as part of Windows. So (theoretically at least) you don't need anything else.
However even Microsoft have admitted that their stuff isn't as good as they'd like it to be, so you might want to consider using the free version of Avast (which is what I use) instead:
https://www.avast.com/en-gb/index
That gets your anti-virus solution sorted out but some people are also critical of the firewall that's built into Windows. If you'd prefer to use a different free firewall (that offers better '2-way' protection), Comodo will meet your needs:
https://www.comodo.com/home/internet-security/firewall.php
That just leaves anti-malware provision (which even commercial products such as McAfee, Norton and Kaspersky tend to be very poor at dealing with anyway). Simply download and install the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: When installing it, take care to remove the tick alongside the option to take a free trial of the Premium version). Because it's 'run on demand', rather than 'run in the background', you just need to remember to activate it periodically (perhaps once per fortnight) to scan for any problems.