I built a system about a month ago with an Intel i5 2500K, and I used the stock cooler cooler without paste (I was told paste isn't required unless you overclock). My PC ran fine for a month, it would go up into the 80-90 degree Celsius range with graphics turned all the way up.
Now today, I turned my PC on and after about 5 minutes it turned off suddenly, and wouldn't reboot. After leaving it off for a few minutes, it would turn back on. After doing this 2-3 times, I checked the CPU temps and right after turning my PC on. With it running idle, my CPU was sitting at 85-90 degrees Celsius, and my PC would shut off if I tried to open anything. I've checked and all of my fans are running.
Are the intel stock fans always this bad? Is there anything I can do to lower my temperatures, or should I go and spend $50 on a solid fan, heatsink, and paste?
I /facepalmed when I read this "I used the stock cooler cooler without paste (I was told paste isn't required unless you overclock)". Who told you this? This is 100% false. You ALWAYS need thermal paste. ALWAYS.
Those are some pretty high temps you hit there. I am truly surprised that your pc ran for a month like this. Those temps are not good at all for your cpu, and you better hope that it still works.
So here's what you do. You must add thermal paste, no matter what. Now, do you wish to over clock your cpu at all? If not, then the stock cooler should be fine. If you plan on overclocking(I would assume so since you paid extra for the K version of that processor), pick up and aftermarket heatsink, and once again, you must add thermal paste whichever option you go with. I'm not an expert with after market heat sinks, but off the top of my head I know this one is pretty decent http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I don't know who told you the cooler does not need thermal paste. They all need whether or not you overclock. It should not be something you would need to apply yourself though. The bottom of the stock heatsink should have a few grayish stripes, that's the thermal paste.
It could be possible that the heatsink has not been properly installed. Since it has been a month, you can't simply remove the heatsink and place it back on the CPU. You need to clean off the thermal paste from both CPU and the heatsink with rubbing alcohol that is at least 90% alcohol (less impurities and better for paste removal).
You will then need to apply a new layer of thermal paste. I prefer good 'ol Arctic Silver 5; it's not the best thermal paste around anymore, but it should be available at Bestbuy if you don't feel like ordering online and wait for it to come.
Generally speaking, Intel stock heatsink and fan is enough to keep your CPU cool.
1) The stock fan should be ok. It comes with three grey stripes of thermal material pre applied. That should be sufficient.
Under load, it will spin up and be noisy, but effective enough if you are not overclocking.
2) Incorrect installation is probably at fault here. With the power off, see if you can wiggle the cooler. If it is not solid, then one or more of the push pins is not solidly set, and you must reinstall the cooler. You will need to remove the cooler, clean off the old paste with alcohol, and reapply new. AS5 or MX2 are good. The trick to installing the pushpin type is to psh nown simulataneously on diagnoal pairs. Ideally, you should do this with the motherboard out of the case so you can check the back side and verify that all 4 pins are fully through and locked.