Vista users should be able to install Npcap driver normally now.
For Win7 x64 users:
If you still get the pop-up window that said
Windows requires a digitally signed driver (or get error
577 when executing
net start npf), please try these steps:
1) Install the following update patch successfully. (Windows SHA-256 certificate security)
KB3033929 for Win7: https://technet.microsoft.com/en-us/library/security/3033929.aspx
This step should requires reboot.