I have not tried this myself, but heard from a colleague that it is very good. It can function as a stand alone application or as a Photoshop plug-in.
Download from here:
Polaroid Dust and Scratch Removal Utility
Taken from the readme file:
"Polaroid Dust & Scratch Removal Software is a standalone Windows application or Adobe Photoshop® plug-in filter application allowing you to selectively remove dust and scratches from a digital image. This software utility is an easy to use application that detects defects and cleans the image while retaining and preserving the original image integrity. Works on TIFF, JPEG, or BMP images."