Microsoft Security Essentials (MSE) is Microsoft’s free anti-virus software. It is designed to protect against malware and viruses on Windows XP, Vista, and 7, but in Windows 8 it has been discontinued and merged with Windows Defender.
In truth MSE has never been a robust anti-virus, receiving widespread criticism for covering the basics of malware protection and nothing more; we have always recommended that you should replace MSE with a more effective antivirus.
Sadly, Windows Defender does not fare much better than its forerunner. It looks and feels almost exactly like MSE, and like its predecessor offers little more than a barely acceptable minimum level of protection. You should unquestionably download and run and third party antivirus suite. Avast and AVG are popular, but if you’re not sure there are lots of excellent free antivirus alternatives, along with several sites that offer reliable antivirus reviews. You could even try a cloud-based antivirus program.