Can't remove disconnected (ghost) network device

Ok, I'll try it in safe mode (haven't thought of that yet ) but I can only do it on Monday. I'll report how it went...

Ok, dear desperate google visitor from the future, here's how I solved it:

Safe mode doesn't help at all, by the way, it gives you the same error if you want to remove the NIC via DeviceCleanup (or it does nothing at all when trying to remove it via device manager).

The solution was to uninstall the Intel driver software ("Intel Network Connections", also called PROWinx64.exe) via its uninstaller and then reboot the PC. After the uninstall, both NICs (the real and the ghost one) disappeared and after the reboot the real NIC was normally detected by Windows using the generic ethernet driver by Microsoft - and voilá, the LAN connection appeared with the default name "Ethernet" (no 2 or anything) and there is no more ghost NIC in the device manager. Afterwards, I've reinstalled the latest intel driver and that's it.

So it was the Intel driver software all along! Mind you, after changing the mainboard the first thing I did was to run the PROWinx64.exe setup (latest version) to make sure to get the latest driver version as well as to resolve potential issues like that (and also installed other chipset drivers for the new mainboard). But apparently that's not how Intel drivers work - you have to completely uninstall it first, then reboot and then reinstall it. Thanks for the user friendliness, Intel!