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The 1 Long + 2 Short indicates a GPU problem ; try another GPU. Also, a weak PSU can sometimes cause the problem, but it typically is the GPU. Also, if there's a TV Card then try booting without it and if your PC has an IGPU {Integrated GPU} try only that as the video ot and remove any added GPU.
HP Pavilion Media Center TV m8120n http://h10025.www1.hp.com/ewfrf/wc/product?cc=us&lc=en&dlc=en&product=3440615