Recently I've worked on numerous computers that have been infected with a very nasty fraud virus that can be difficult to remove. Most of these infected computers did have anti-virus software installed and the software either wouldn't pick up on the virus, or became infected itself! Of these computers, the ones that have had anti-virus programs installed that didn't work were: Norton, AVG, and Microsoft Security Essentials. While it's often a good time to keep same-brand software for compatibility, I can't really suggest using Microsoft Security Essentials, even if it is free. There are so many other options out there, even free ones, that have a proven reliability.