After installing SmartSDR for Windows, you may encounter a warning that the kernel-mode WDM DAX driver does not have a valid digital signature and Windows renders the driver unavailable.
First off, the error message is incorrect. All SmartSDR drivers are signed with valid digital signatures.
The PC that is having problems with errors indicating missing digital signatures is having the issue because it does not have an updated root certificate. Normally, if you install all mandatory and optional updates using Windows Update, this will resolve the issue.
However, there has been reports that some systems (Win7 and Win 8.x) are not getting their root certificate updated properly. If this is the case, then I recommend installing Microsoft Hotfix KB3004394
This hotfix is not an updated root certificate but is a patch to ensure that the PC will get the updated root certificate. Once this is installed, it may take 12-36 hours before the root certificate updates on the PC.
If you do some investigations on this hotfix, you will see that when it was initially released (Dec 2014), it caused a lot of issues with people's PCs. This hotfix was subsequently re-released in Feb 2015 and it works. The link above is for the corrected hotfix.
However, if people on Win7 and Win8.x have trouble installing KB3004394, then follow this procedure to correct it.
1.) Check your MS Update history and see if KB3004394 is installed. (http://windows.microsoft.com/en-us/windows/which-windows-updates-installed#1TC=windows-7)
2.) If it is, manually uninstall it.
3.) Check to see if KB3024777 is installed on the PC. If not, install KB3024777 (https://support.microsoft.com/en-us/kb/3024777)
4.) Now install KB3004394 (https://support.microsoft.com/en-us/kb/3004394)