To boot to the ShadowProtect Recovery Environment CD on a Dell Precision Tower 3420 you need to change the BIOS to enable Legacy Boot and disable Secure Boot.
To enable Legacy Boot on the Dell Precision Tower 3420, you have to turn on Legacy Boot Rom (I think it was called – it’s the option in the menu item below the boot options). Then when you click the Legacy Boot option it will jump back to UEFI Boot. Click Apply and it should allow you to select Legacy Boot and disable the other option. Once you have disabled Secure Boot (in another part of the BIOS) you will then be able to boot to the ShadowProtect Recovery Environment.
Naturally, you then have to set the Dell PC to boot to the CD (or whatever media your ShadowProtect Recovery Environment is stored on). You can do this as a one time boot by pressing F12 on the Dell 3420 and selecting the boot CD.
Addendum: Apparently some BIOS’s also have an option called Safe Boot, which can make you unable to boot to the ShadowProtect Recovery Environment CD.