After installing the software and drivers for our AT&T air card on a Windows XP machine I was unable to launch the connection manager.
Everytime I launched it we recieved an error message stating “application failed to initialize properly (0xc00000005).”
I tried un-installing and re-installing and downloading a newer version from their website all to no avail.
Finally after several Bing searches (yes I said Bing, not Google) and trying a few different “fixes” I had success.
I’m not sure which of the final 2 fixes actually did the trick but they were:
Open command prompt and type:   sfc /scannow
This causes the system file checker to run and fix any issues with system files.
I’m pretty sure that’s what fixed it but at the same time I also enabled and disabled the Data Execution Prevention setting under system properties/Performance/Data Execution Prevention tab.
By default it’s set to “Turn on DEP for essential Windows programs and services only” I changed it to “Turn on DEP for all programs and services except those I select:” hit apply, did not reboot and then changed it back and then rebooted.
After rebooting the AT&T connection manager launched but got hung.  I rebooted again and ever since it’s been working fine.

During the process I also ran to following 2 commands, I don’t think they helped but I put them here since I did it during the troubleshooting:
Note:  I vaguely understand what these do and don’t suggest you do this unless you know what you are doing:
CACLS %systemroot%\System32\*.dll /E /G BUILTIN\Users:R
CACLS %systemroot%\System32\*.ocx /E /G BUILTIN\Users:R