I think it is advisable - though it does depend on browser security and browser and other habits - for those on Windows to run an anti-malware program. So the question becomes, which?
Windows defender on Windows 10 has has good reviews recently. So, if one has Windows 10, then Windows Defender may be all that one needs. Yet, if one needs (because one in on e.g. Windows 7) or *feels* the need for something else, then there are a fair few products that have received good reviews, though there is more than one thing to take into account. For, there's price, performance impact, detection, false positives, how much the program bugs the user, etc. And one has to decide whether one wants 'realtime' protection, or realtime protection and something one runs occasionally, or both. Good resources for investigating such things are: http://www.av-comparatives.org/
Perhaps, though, Happyman, you want advice that is more directly to the point. So: if you think you need more than Windows Defender, then: BitDefender has been highly rated on most measurements recently, though the free version is a bit intrusive; Avast, Eset, Avira, Sophos (and I list these only in alphabetical order) are all at least decent (though perhaps not better than the Windows 10 version of Defender, and free versions of some of those might irritate).
Myself, on Windows, I run Windows Defender (in 'realitime', i.e. always on) and have a program called 'SuperAntiSpyware' that I run occasionally. Note that more important, probably, that anti-malware programs are security browser extensions; get Ublock Origin (or equivalent) and, perhaps, Privacy Badger, for your browser. Also, what you really want if you get infected by malware (which happened to me once, I think - I wasn't entirely sure) is really good *backups*. For then you can, so to speak, nuke your existing installation and replace it with a backup. I could advise on backup programs if you like.
[EDITED to fix a typo.]