Yes, 100% certain (since I'm the one who built it and installed the OS in the first place) it was Windows XP x64 since I have the old hard drive I can just reconnect it and boot it up, here's a picture:
Like I said this was not an upgrade but I swapped in a brand new hard drive and did a clean install so no I didn't run the upgrade assistant because I'm not upgrading and this system meets the minimum hardware requirements. This is what Microsoft lists as Windows 8.1 minimum requirements:
- Processor: 1 gigahertz (GHz)* or faster with support for PAE, NX, and SSE2 (more info)
- RAM: 1 gigabyte (GB) (32-bit) or 2 GB (64-bit)
- Hard disk space: 16 GB (32-bit) or 20 GB (64-bit)
- Graphics card: Microsoft DirectX 9 graphics device with WDDM driver
and this is what's in this computer:
- AMD FX-4100 3.62Ghz Quad Core CPU
- RAM: 8GB
- Hard disk: 1TB Western Digital Black Edition Drive
- Zotac NVidia 560 TI
so this computer more than meets Microsoft's minimum requirements.
Yes, the drivers are old since if you select Windows 8.1 64bit the Asus website doesn't even show you chipset drivers and you have to select Window 7 x64 and all you see is drivers for Windows 7 from 2010 (6 years old is old in computer / technology years). I'm looking at the same URL you list and I don't see any chipset drivers from 2014.
You say "But more importantly, because notebooks tend to be very proprietary, it typically is best to use the drivers from the notebook maker and not the OEMs." but this is not a notebook and I never said it was. The M5A78L-M LX Plus is a standard Micro ATX form factor motherboard so notebooks and notebook manufacturers have nothing to do with this.
The M5A78L-M LX motherboard uses an AMD chipset and there should be no problem installing the latest available AMD chipset driver package. But as I said the driver package installs (no errors or problems during the install) however none of the hardware after reboot shows that the OS is using AMD drivers and is still using generic Microsoft drivers for all the the hardware.