Today, my PC began shutting itself off after 10-20minutes, when I tried to boot in safe mode, it crashed with a long beep.
Now it turns on for maybe 5 seconds with only a red light before shutting off again. I tried changing the ram and psu, no change. I gave the CPU a good clean and now it makes a different sound and stays on a couple of seconds longer before turning itself off so I am assuming it's something to do with the CPU.
Please provide system specs, ie MB/Ram/GPU/PSU (orginal and 2nd one you tried).
Try a barebones system onfig - No HDDs, No DVD drives, one stick of memory (IN THE PREFERED SLOT), Have you tried clearing your CMOS (probably have, but had to ask).
If above allows you to get to bios, try reseting to failsafe and start adding back componets you removed.
Since the "cleaning" changed symptems - recheck/reset power connections on Motherboard, and GPU.