The lightest from capable AVs are BitDefender AV Free and Forticlient. You can for sure rely on Windows Defender but I can't call it the good defense.
As for me I use no AV at all building my defense on:
1) using alternative software - browser not from Big Four (K-Meleon), altenative e-mail client (Sylpheed or any other one), PDF-reader (Foxit or Sumatra), messenger (Trillian) etc. This dramatically reduces actual attack surface.
2)filtering web-content - ads, malware sources etc. (AdBlock Plus, K9Web Protection, Forticlient inbuilt web-filter, OpenDNS etc.)
3)using some monitoring tools for registry, autostart and so on.
You can also set built-in Software Restricion Policies or Applocker if present in your Windows edition - to get white list of trusted applications or create rules for apps run (it's a very good idea to set some foldser to saving web content and restrict any app run from there as well as from system TEMP folders) or simply install EMET or Malwarebytes Anti-Exploit to harden your setup from exploits.
Also there is very useful application - Secure Folders, which allows to set effective but more user-friendly restrictions for app run.
So the bottom line is - when you are limited in resources you should combine different 'lightweight' solutions creating a multilayer security config.
