As stated above, microsoft can give off false positives sometimes. this could also be a case of a bad Ad showing up. Usually this is an issue with flash and malware piggy backing an Ad. If you are browsing and get a warning, let me know which ads are playing on that screen by getting screenshots, along with your region location as well.
again, as stated above, i would give your computer a good deep clean of malware and do a virus scan. there is malware that post even as microsoft warnings to get you to download random stuff telling you that its helping your computer, but in reality, it is in fact. malware. make sure you are back checking those random microsoft warnings.
i suggest using official microsoft security essentials to further assist you on this. also, lots of free malware software out there that will help you out as well.