Once you've scanned your photo and have it on your computer open it up in Photoshop and duplicate it.

1. Dust and Scratches

Next, go to Filter>Noise>Dust and Scratches. This brings up a window with two slider controls: Radius and Threshold and a preview window.  If you only adjust the Radius slider the dust and marks vanish but the edges of the image soften making it look like you've smudged it. However, if you adjust the Threshold too you'll see the detail slowly come back. Don't take the slider too far though as the photo will become blotchy. Adjust both sliders until the dust is removed but the sharpness and edge detail remains then click OK once you're happy.

2. Lasso Tool

If you still have a few scratches that are visible select the Lasso tool and make a rough selection around the problem area. Once you have a circle of 'running ants', select the Dust and Scratches filter once again. Adjust both sliders until the marks are removed but there's still enough grain left to ensure the adjusted patch blends with the rest of the photo. Take too much away and you'll have an obvious smooth patch that stands out from the rest of the image.

Finished Image: