While decent Windows Defender is not a top contender for AV products. I would also point out that it not detecting anything isn't necessarily a good thing it can simply mean it can't detect a particular malware sample. Avast and AVG are far better free AV products. For a good run down of WD you can read this. They do lab tests as well as show the big four independent testing groups ratings for lots of AV products.
Microsoft Windows Defender Security Center
Microsoft Windows Defender Security Center