PC doesn't boot, even when fixed returns to this after a while. Help needed

[BevanPage]

I feel I have a very unique problem. My Pc will power up (the blue power LED comes on and I hear the fans), but the PC doesn't boot. This happened originally after I tried to fix a problem with Windows 10 where the start menu doesn't work at all. I believe the problem happened first when I tried to boot in safe mode. The Hard drive LED on the case used to come on for a second or two when this problem started, but now it doesn't at all. Another thing that has changed is when this problem first started the PC would constantly restart itself every few seconds on an endless cycle, however after the original time this problem appeared it has not done this. The way I found to fix it was to clear the CMOS on the motherboard (move the small connector from one set of pins to the other) and to remove the motherboard battery and replace it after powering up to clear the CMOS. After doing this it would usually fix my PC. However this problem has happened roughly 5 times now in the past two months or so - the PC powers up, but the hard drive light does not come on and it does not boot. I'm confused by this as I'm unsure of what the problem is: the HDD, MOBO or something else? When I do the fix as described earlier everything works fine, but eventually my PC returns to the problem. Any help would be greatly appreciated. My PC specs are as follows: http://support.hp.com/gb-en/product/HP-Pavilion-p6-2400-Desktop-PC-series/5330783/model/5403179/document/c03823591/

 

[Outlander_04]

I suspect you have a faulty power supply , or perhaps the power cable to your hard drive has some corrosion .
Hard to prove one way or another without trying a new power supply

 

[BevanPage]

I suspect you have a faulty power supply , or perhaps the power cable to your hard drive has some corrosion .
Hard to prove one way or another without trying a new power supply

Thanks, with Christmas coming up I was curious as to what new parts I should buy. However I don't understand why resetting the CMOS and MOBO would fix this if it were the problem. I was considering a PSU upgrade anyway as mine is only 300W at the moment, and I should be getting a new graphics card. Thanks for the speedy reply, I'll try to test a functioning power supply.

 

[BevanPage]

I suspect you have a faulty power supply , or perhaps the power cable to your hard drive has some corrosion .
Hard to prove one way or another without trying a new power supply

What would you suggest if this is not the problem?

 

[Outlander_04]

I suggest eliminating the psu as the source of the problem . Just borrow one from another computer . You may not even have to install it in the case . Just plug in and boot

 

[BevanPage]

I suggest eliminating the psu as the source of the problem . Just borrow one from another computer . You may not even have to install it in the case . Just plug in and boot

I have tried a new power supply but nothing has changed, any other suggestions? And recently it seems when it eventually fixes after me turning it off and on enough times, the problem instantly returns after me shutting down the system