Early this year, Windows changed the way they sign and verify drivers. From March 2017, Windows enforces using SHA256 based signatures (instead of SHA128) . Due to this change, we also signed our drivers from Microsoft with SHA-256.
Some versions of Windows 7/8 had not support for SHA-256. Microsoft has provided updates to support SHA256 on these OS.
I suspect that your PC does not have an update and hence cannot read the SHA-256 signature. Could you check that you have the following update https://technet.microsoft.com/en-us/library/security/3033929
I hope the driver installation succeeds after you update your Windows.