Try soft-resetting your device and then trying the installation again. Make sure to say "Yes" if asked whether you want to install software from an unknown publisher.
If you are still having trouble installing after just a soft-reset, try the steps below to minimize the number of applications running on your PDA before installing or uninstalling:
- Turn your device off, wait 30 seconds, then turn it on again and perform a soft-reset
- Place your PDA into 'flight mode', or otherwise turn off the Phone radio, WIFI radio, or Bluetooth if so equipped
- Stop all running programs except ActiveSync. On a Pocket PC, go to Start -> Settings -> System -> Memory -> Running Programs. On a Smartphone, use your favorite task manager program, or the one included with your phone
- Temporarily disable any third-party Today screen plug-ins on your Pocket PC, or home screen plug-ins on your Smartphone
