Sorry i'm not too technical but i know a fair bit so i'll try explain as best as i can... (Can't remember all of my PC specs, kinda old)
NVIDIA® GeForce® GTX 260
Intel Core2 Duo E8400 @ 3.00GHz
OCZ 2 x 2GB Ram DDR2 Fatal1ty Edition
MSI P7N Series (Platinum i think)
Cooler Master Silent Pro 1000W (Replacement after last power supply killed itself)
Windows 7 Ultimate 64 bit
Oh, skip to "What I Know So Far" bit if you want to skip the story of how my PC went from fine to broken...
My problem is that my computer boots up fine and does everything as it should. After so long, (depending on how many programs i decide to open up on startup) the monitor turns off. I have used itunes a couple of times while it has done this and the sound does a loud distorted sound then i turn off the computer by having to hold the power button.
My computer used to work fine until one day my friend decided he wanted to try his CPU out in my computer, so i let him and it worked fine. We then reassembled my computer and the computer worked but after about maybe 8 hours of it running it started having the problem that i originally stated. Around this time it was time to go to bed, so i went to bed. Next morning turn it on, it works fine for about 3 hours then has the original problem again and every time from then on, it seemed to shorten the time. Since i blamed it on my friend for messing with my PC, he decided to try and fix it so we cleaned out the PC of all its dust, cleaning each and every fan and all the components. This didn't help much. So then i did some googling.
What I Know So Far:
Basically, when the computer does turn on for that short amount of time, i have a habit now of opening up task manager and going on performance. What i can tell you is that the Memory Usage is always at around 1.89GB and also the CPU usage spikes a lot, especially when loading games and reaches all the way up to 100% but it spikes a lot.
I first read that it might be the CPU cooling, so i took off the Intel CPU cooler and found out the thermal paste was all dried up so I cleaned it and applied new thermal paste it plentiful amounts... Didn't work
Then read it might have something to do with the RAM or the slots, so i tested the RAM individually and all the slots, which wasn't the problem.
Then read it might be my graphics card or the slots, so i tested my bros spare graphics card and found out that wasn't the problem, neither were the slots.
Today i spoke to the IT guy in town while picking up some ink cartridges and he said that it is a common problem with intel processor fans and that i needed to make sure it was in correctly, found out it wasn't so today i fixed that and applied some more thermal paste. The CPU usage still spikes but it actually manages to reach 0% for a little bit, before it starts spiking again and per usual eventually kills itself.
My bro says that he used to have this problem and then got water cooling, then problem solved. A little but reluctant to spend £50 - £60 on CPU water cooling to find out that it might not be the problem.
I personally believe one of my RAM's have died and that i do probably need cooling, but i don't trust myself to come up with an accurate answer for my own problems 'cos im not very tech headed i guess.
Thanks in advance if you have any suggestions other than buy a new PC as i really don't NEED a high performance PC anymore and this one does what i need it to, plus most components... well nearly all components aren't for sale commercially anymore.
Thermal paste must be properly applied, too much or too little and you'll have the same problem. I hope you used the silver paste not the white one.
If you got no errors on memtest (8 passes overnight) your ram is perfect.
Have you checked your power settings in control panel ? Take monitor off sleep mode.