Files that have an .atx extension store ArcGIS 8 attribute indexes. An .atx file is created for each shapefile or dBase attribute index created in ArcCatalog.
ArcView GIS 3.x attribute indexes for shapefiles and dBASE files are not used by ArcMap or ArcCatalog. A new attribute indexing model has been developed for shapefiles and dBASE files.
ArcObjects Help provides a sample tool to convert ArcView GIS 3.x attribute indexes to ArcGIS 8 attribute indexes. See ArcObjects Developer Help > Contents > Samples > ArcCatalog > GxTables > Convert AV3 to AV8 Attribute Indexes.