A problem was identified with the installation of the ArcGIS 9.3.1 Desktop Download Layer Package Patch.
In some cases, after applying this patch, it would cause ArcMap or ArcCatalog to crash on startup. An updated version of the patch has now been posted to the ESRI Web site.
If you downloaded and installed this patch on June 23rd, 24th, or 25th of 2009, it is recommended that you re-download the new version and apply it again. The new patch can be applied on top of the old one, and this should resolve the issue.
We apologize for any inconvenience this may have caused you.
- Mike H., User Advocacy Group, ESRI Support Services