The GroupWise Client 6.5.x on Windows 2003/Terminal Services shows its splash screen, then it disappears. Either the splash screen will appear, and an error message will appear in the Application log, or there will be no Application log error.
Make sure Data Execution Prevention (System->Advanced Tab->Performance/Settings button->Data Execution Prevention) is set to "Turn on DEP for essential Windows programs and services only". I tried the second (exception) option to no avail. Save yourself some misery!