I don't really use Windows Defender myself. I have read it works well enough, but it is made by MS. Not that I have anything against Microsoft, but dealing with security threats is not what they are known for and not a primary concern for the company.
Spybot works very well for me. I install it on all of my computers, and keep it up to date, and scan at least monthly. I like it because it is free, so the old excuse of not getting updates because a subscription expired is not valid. If you want a pay solution Spysweeper works well.
Then you need an antivirus solution as well. I like AVG free edition, again free updates means no excuse to not have an up to date definition base. It also works very well as far as I am concerned, and I have used it for years without issues. As far as Pay versions go I am a fan of TrendMicro and Kaspersky. Both work well and are light and unobtrusive.