As a single driver, I don't know.
Where do I get this driver?
It is part of the chipset driver package for AMD/ATI chipsets.
Go here, pick your chipset and download, install.
If the SMBus driver doesn't install, look for it manually in the ATI folder on your system drive, it will have it's own folder. To install manually, go to:
Start>Settings>Control Panel>System>Hardware>Device Manager>
Look for yellow exclamation mark under "System Devices" in front of your device.
Right-click, choose "Update Driver" and "Install from a specific location."
browse to the ATI (or AMD) folder the chipset drivers were expanded to and navigate to the SMBus folder. Click Ok. Driver gets installed. You need to restart Windows.