Ethernet issues - and one more thing

So, here’s the situation.

I bought a PC with an MSI B450 Motherboard, and it arrived with Windows 8.1 already installed—presumably just for a configuration test. After I installed Windows 7 (twice) on it, the system won't recognize the ethernet cable when I plug it in. There isn't even a link light on the port to show a connection between the connector and the motherboard. During the Windows installation process, it doesn't even prompt me to check for an internet connection; I assume it just skips that part because it can't detect anything at all.

The cable itself is fine. I bought it two months ago, and it works perfectly when I connect my laptop. Even after failing to get it working on this desktop, it connects to the LAN on my laptop without any issues.

I checked the Gateway settings to make sure access to other devices wasn't restricted, though that usually only applies to WiFi, but everything seems allowed.

I tried using the driver from the motherboard's CD, as well as a newer version I downloaded from the internet via my laptop and transferred over using a USB drive. I tested both on both versions of Windows, but nothing works. I even tried disabling EEE and Green Ethernet in the internet settings, since I read online that those can occasionally cause trouble, but that didn't help either.

In the BIOS, I verified that the Intel LAN Controller is "Enabled." It is. I tried toggling it off and back on, but no luck.

Device Manager doesn't show any warnings; it claims the device is working properly.

However, Windows Troubleshooting says the cable is disconnected or faulty.

Everything is plugged in exactly where it should be. Physically, there's nothing suggesting a problem with the LAN port.

Aside from the essential drivers, the only other thing installed is the SetPoint driver for my wireless keyboard and mouse (Pure Military). Both were downloaded from official sites for Windows 7 64.

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UPDATE: As of today, it won't even turn on, let alone reach the BIOS. When I try to power it up, a white LED near the chassis power button blinks, and a green LED on the motherboard (near the bios flash update area) stays lit for a few seconds before dying once the power is cut. On the monitor, right after the AOC logo appears during startup, it briefly displays "no signal" and then goes dark. The fans spin, and I can hear the CPU cooler, but I don't hear the SSD (I assume because it never reaches the boot stage). It's almost like it's stuck in sleep mode. Restarting and power cycling don't help.

Any ideas? Aside from the obvious thought that the shop built this and I shouldn't touch any hardware because it's brand new, which would lead me to just return it this Monday... Is it actually possible for a brand-new motherboard to be defective?

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Specs:

Motherboard: MSI B450 Motherboard (I wanted the DS3P, but they were out, so I took this one based on a forum recommendation)
CPU: FX 8300
CPU Cooler: LC-CC-120
GPU: EVGA GTX 970
PSU: Coolermaster VS550M
RAM: Corsair Vengeance LPX 2x4GB 1866MHz (CL10)
SSD: Kingston HyperX Fury 120GB
HDD: None for now
Case: Antec GX505, fitted with 5 Antec TrueQuiet 120mm case fans

It is entirely possible that some new component failed on you, so there is really no point in overthinking it; if they were the ones who put this rig together and it’s still under warranty, why not just haul it back to them for every little hiccup before that coverage expires? 😁
By the way, you aren't going to hear your SSD making any noise, because they are famously silent.

When I power it up, I hear these two beeps. I can't tell if it's just the usual startup rattle or some kind of alert sound. I guess my ears are a bit rusty since I haven't owned a desktop in ages. 😁

Here’s a link to a quick video where you can hopefully hear the PC booting up (uploading the whole thing would take five hours at 768kbps, lol):

You might have to listen closely to catch it. 😁 damn.

And here are some shots of that green LED on the motherboard:
Sorry about the image size; I forgot to make a thumbnail. 😁

As for the white LED by the power button on the front, it turns on during startup, then shuts off, blinks, and goes dark again. I didn't really check if that's normal, but I figured I should mention it.