Hi zakir_yo & welcome to Windows 7 Forums
This is because the driver is unsigned and the driver signature enforcement in Windows 7 can only be turned of at risk it was possible to turn that feature off in Vista, but due to new kernel security updates, it is impossible to turn it off without degrading security and stability of your operating system or putting your master boot records in risk by using other software alternatives. But there is a small freeware utility that can enable you to use such drivers (Driver Signature Enforcement Overrider)
Link >
http://www.blogsdna.com/2775/install...-overrider.htm
I would stress you only use this for your Modem Driver.