Go to Avermedia’s website → Support → Legacy Products. Search for “A857.” You may find a beta driver for Vista 64-bit, which can sometimes work on Windows 7 64-bit if installed in compatibility mode.
There is no official, signed Avermedia A857 driver for Windows 7 64-bit . Any working solution will be unofficial and may break with Windows updates. If TV viewing is the goal, consider a modern USB TV tuner (e.g., Hauppauge, AVerMedia H830) with native Win7 64-bit support. Driver Avermedia A857 Windows 7 64 Bits LINK
The A857 uses the NXP SAA7131E chip. Sometimes drivers for the LifeView FlyTV series (same chip) work. You may need to manually force the driver via Device Manager → Update Driver → Browse → Let me pick → Have Disk. Go to Avermedia’s website → Support → Legacy Products