.aco is just the file extension of your color swatch files. Depending on which version of Photoshop you have, the default .aco files should be in:

C:Program FilesAdobePhotoshop CSPresetsColor Swatches

or

C:Program FilesAdobePhotoshop 7.0PresetsColor Swatches

You can select from a complete list of the default color swatches when you click on the swatch options triangle as seen in image.