If it is in webbrowser it is fake alert and ignore it. If you want to check that you have Anti-Virus or not go to control panel and chose Windows Security Center / Action Center and see if it has Anti-Virus or Anti-Malware and if it is on. In there it shows that you have or you haven't Anti-Virus and is it on or off or out of date.
If you don't have Anti-Virus I strongly recommend you to get one.