Home build, but together about two years ago.
Motherboard: MSI 870A-G54 AM3 R
CPU: AMD|PH II X4 955 3.2G AM3 RT
Video Card: VGA SAPPHIRE|100283-3L HD5770 1G RT
PSU: ANTEC| NEO ECO 520C 520W RT
Memory: 2Gx2|CORSAIR CMX4GX3M2A1600C9 R
Has worked mostly flawlessly since built. Several days ago, the system went into complete shutdown unexpectedly while admittedly being taxed more than usual. Streaming MLB.tv and Pandora radio simultaneously while encoding MKV video into MP4. After system shut down, the LED in the power button remained lit, flashing. I held it down for a forced complete shut down. Moments later, I tried rebooting. When attempted, the LED power button began flashing but nothing happened otherwise (did not notice if fans were spinning). I unplugged the PSU, plugged it back in, then re-attempted. Pushing the power button yielded absolutely nothing.
Search so far:
Opened system and performed paperclip test on PSU--successful, internal PSU fan spun.
Removed every component from Motherboard, powering only the Mobo itself (also tried with one memory stick and with video card along with a memory stick). When powered up, PSU and Mobo fans moved slightly, then nothing.
Borrowed friend's PSU which worked in other systems and tested with absolutely no results. The PSU though only had capacity for 6 and not 8 on the uhh... smaller power input for the Mobo.
Removed Mobo internal battery and put back in and attempted all tests again with no results.
After Google-ing as much as possible, I have not come up with conclusive results and have been unable to locate a Motherboard Speaker at my local Radio Shack. Are there more telling steps that I am missing? Is it rash to assume that it must be my Motherboard? Could there be something wrong with the CPU? If I do wind up replacing the Motherboard (and potentially CPU), can I "plug and play" it without having to reinstall Windows? Were my tests with PSUs not conclusive enough?