Interesting. I am no expert here, but I think this problem might be a red herring. For various reasons unrelated to your problem I have had to reinstall Windows XP and Windows 7 several times on my system, and each time I have had to reload the drivers, so I am fairly familiar with what happens. On initially loading Windows XP the SMBus isn't listed, but there are a lot of unrecognised devices in device manager. When I load the SMBus drivers, one unrecognised device dissappears and the SMBus appears. With Windows 7 the option to load the SMBus driver from the motherboard CD isnt' available. I'm currently in Windows 7, so I've just compared my SMBus to yours and I've got essetially the same message as you will see in the first screen shot below. Now take a look at the second one. As you will see, when checking the driver details for the device it states that a driver isn't needed. My suspicion is that all the so called driver files do is enable the operating system to find the SMBus, then the operating system takes over and does what configuration it needs to be able to use it, but after that no actual driver files are needed. I suspect if you check the driver details on your PC you will find that your PC, like mine, is happy it doesn't need drivers to use the SMBus and, in fact, the PC is already using the it to its full potential.
