Here at Liquidstate HQ, we have a simple HP Microserver N40L running Microsoft Windows that acts as our fileserver. I recently made the mistake of trying to upgrade to Windows 8.1. What a drama.
After two days of googling, fiddling, and general hair pulling, it seems there's a fairly simple formula to getting this all working.
Step 1 - Apply all Windows 8 Updates
The first thing you need to do is make sure your Windows 8 installation is up to date with all recommended patches.
Step 2 - Upgrade the BIOS
HP released an updated BIOS (Ver. 2013.10.01 (A)) that fixes issues with using the onboard network port with Windows Server 2012 and Windows 8.1.
If you don't upgrade then you'll find that Windows 8.1 will take a very long time to install and will ultimately then get stuck on first boot at the "Getting Devices Ready" stage. For me, it just hung there indefinitely at 25%.
The update is available here, but unbelievably it requires an active warranty, HP Care Pack or support agreement linked to your HP Support Center profile. For the time being though, you can download it directly from here without any of the faff though.
Step 3 - Install Broadcom NIC driver
You'll most likely also need to install the latest NIC driver from Broadcom, rather than rely on the one that was included with Windows 8. You can get it here.
Step 4 - Install Windows 8.1
You should now be in a good position to download Windows 8.1 from the Store and install it. Good luck!