You can control the behavior of Dr. Watson by editing the registry at:
A data value of 0 in this type REG_SZ causes the system to display a message box notifying the user when an application error occurs. A data value of 1 (the default) causes the debugger to start automatically.
You can disable the good doctor by deleting the AeDebug subkey. If you wish to re-enable it, type drwtsn32 -i from a command prompt.