If you are using Windows 7 or 8, try Microsoft Essentials. It is free and seems to work flawlessly with Windows.
Bump on that.
Have been using Windows Essentials for 2 years, XP Pro machine and Win 7 machine.
Run Malwarebytes and Kaspersky tdsskiller once a month to make double sure.
Microsoft Essentials hasn't let anything through yet.
Works slick and doesn't eat up processor time.
Had McAfee before, just became too much bloatware for the older machine, would almost drag it to a stop.