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#11
Literally two Windows machines (this one and another one), both with NICs plugged into a switch/router, and able to communicate with each other via the network. It doesn't matter whether you "share" files between them presently.
Unfortunately, that's the flip side of the "building the PC yourself" coin, and most of us will empathise because we've also been there. It might be the motherboard, or the video, or the PSU, or the processor itself... it's difficult to be certain without the luxury of similar hardware to perform trial-and-error component swaps.
If you feel you've done enough to convince yourself that it's hardware, then you may want to ask the moderators to move your thread to the "hardware" section of the forum. The relevant expertise tends to congregate over there.