I found it to be crap. It constantly nagged me to upgrade to the paid version. I used it temporarily until MSFT came out with a very good and highly rated Family safety which is fast and updates itself. Eventually I was able to install Nortons after finally getting rid of avast, which could only be installed by using an advanced uninstaller after trying several others.
Nortons is free from my ISP up to 7 Computers, I have a free 5 year McAfee on one notebook, as it came with it. another Notebook kept crashing with McAfee and I installed Nortons. No problems with it.
Windows Defender comes with win7 and I run it weekly. It will not run if your A/V or MSFT Security is not up to date.It also will not run on some winXP and will on others depending on the version of the OS.
You can have as many Antimalware progrfams as you wish, Malware anti-malware is great5 and free along with system mechanic.