only ONE antivirus can be run in real time, you can have a malware scanner, and/or a spyware scanner installed as on demand scanners, but not another antivirus.
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http://www.techradar.com/us/news/so...free-antivirus-9-reviewed-and-rated-1057786/1
you can do what I did, try several of the free ones, but make sure that they will still support xp, some will quit support for xp in 2015,
I have been switching back and forth between avast and quiho,
avast: have popups turned off as I hate to be nagged about buying avast, did not install any of the extra items as I already have other programs to do these things,
but it is good at detecting.
360: until it learns your pc, there are many false positives, and you end up whitelisting lots of programs, cannot find a forum to ask questions, and I do not like the fact that it has to be granted an exception to my firewall, (windows firewall)
but scan is fast and it does detect lots of stuff, especially when you download an item that has any PUPs bundled with the download.
bitdefender: tried this one to but I like to have control over options, but this one is very good.