When I build a Windows CE 8.0 image from the workspace and the BSP files provided by Toradex without making any changes to the image Windows CE boots fine. As soon as USB Modem Class Driver is added to the image Windows CE stops booting.
At which point in the boot process does the system stop? You can turn on the debug messages to get some hints on the serial port, or even better hook up the Visual Studio debugger to get more insights about the reason of the boot failure.
Another option would be to extract all the required files and registry entries for the USB modem driver. This way you can use our default image and add the files / registry entries on top. This has the big advantage that you will be able to use our latest OS updates in the future without having to rebuild your own image.