We now require SHA2 digital signature support to use Ninite.
If you're using Windows 7 you may not have this support. The solution is to make sure you have Windows 7 Service Pack 1 installed and then run Windows Update to make sure you have the latest patches to add SHA2 support.
If you'd like to just install the specific patches needed, here is the 64-bit patch and the 32-bit patch
If you keep having trouble there's probably another problem. Please get in touch with us and we'll help you.