Whatever antivirus and security scanner you use, it's always good to be able to get a second opinion. However, you can't normally install more than one on-access scanner because they end up fighting over who will scan each file as it's accessed.
You can, though, use a portable non-installable scanner to run an occasional additional check on your PC to ensure that there are no instances of malware that your inbuilt scanner has missed.
One such program is Stinger, from McAfee. You'll find it at http://www.mcafee.com/uk/downloads/free-tools/stinger.aspx and it's a 15 MB download which works just fine on all recent versions of Windows including 10. It also claims to be able to decrypt certain types of ransomware, so keep the download link to hand in case you need it in the future.
Note that, when you run Stinger, it will only scan a small selection of the files on your disk. For a more thorough scan, use the menu to choose the entire disk before starting the operation. It'll take longer, of course, but is more useful.
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