|When does this message arrive, on bootup, when going online, or what?|
What AV or anti-malware programs are running?
You need to determine what is producing that message somehow in order to proceed. As signatures are mentioned it might be your virus checker, or some anti-malware program. If all else fails (and it happens on bootup) you press the "Windows key and R" then type msconfig in the Run box and temporarily disable any suspects you see in the Startup tab.
Always pop back and let us know the outcome - thanks