Posted a long time ago about the KML-Circles site, which lets you generate polygons/quasi-circles centered around arbitratry points in KML format for Google Earth and Google Maps. Lots of options at that site, perhaps too many if you just want a simple circle. For that, there’s the very-stripped-down KML Circle Generator site. Enter the center point for the circle in lat/long, and either the desired radius in meters or a lat/long position you want the circle to pass through:
Press the Go button, and get a link to the KML file:
Depending on how your browser is set up, clicking on the link will either download it to your computer, or open it automatically in Google Earth. Right-clicking on the link and choose “Save As” is the safest, as it will definitely download the file to your computer. Open it in Google Earth:
Default is a red circle; if you don’t like that, you can always right-click on the KML file listing at left, choose Properties, and change the style to whatever you want:
BTW, that big hole in the ground inside the circle is Meteor Crater, just outside Winslow, AZ.