ArcGIS Desktop 9.3 help offers a new section on Tool errors and warnings, which can be a great help when you receive an error code but are not sure what it means. Search for your error code, or navigate through the 9.3 help index (under Geoprocessing tool reference) for a list of error codes from 1 to the infamous 999999. They offer a description and solution for each error code which can help get you on your way faster. Don't forget to search the ESRI Support Center User Forums as well if you are still stuck - chances are someone else has experienced the same issue and you can pick up some tips.