I agree with the majority of the other answers here. Microsoft Security Essentials is the way to go. You can get MSE as a free download here - http://www.microsoft.com/Security_Essentials/
Since it is free, there is no harm in giving it a try. If you find you like something else better, you can easily change programs.
Also, keep your computer secure with this security checklist for Windows 7 - http://windows.microsoft.com/en-us/windows7/Security-checklist-for-Windows-7
Microsoft Windows Outreach