Blue screen issues on new build

[Imnick]

My brother recently built himself a computer and has been having some issues actually using it, he tells me it used to crash every couple of hours but sometimes as often as every 20 minutes. He tried taking the RAM out and putting it back in just in case it was loose but that didn't really seem to solve anything and also involved unscrewing the heatsink and then putting it back in again.
Since then the crashes have also occasionally been accompanied by a message saying that the CPU has overheated, though as far as I can tell all the fans are running properly and in the right direction and I thought it would be an adequate amount of air flow. I am not sure if this was the original cause of the crashes or not because he only asked me for a hand once it had got to this stage, and I'm sort of stumped as to what to do myself.
When I built my computer it just sort of worked the first time so I have little experience debugging hardware!

I have a suspicion that the fans on his heatsink aren't operating at full power because they seem to be producing an awfully low amount of air (we did check they were facing the right way) but I'm not really sure how to confirm that. Additionally he lives about 20 minutes walk away from me and obviously cannot reliably check this topic from his internet, so if you have any other ideas of what the issue could be and how to solve it then that would be appreciated.

This is the build he is running:

Motherboard: Asus P8Z77-V LK
CPU: Intel Core i5-3570K
RAM: G.Skill Sniper 8GB (2x4GB)
Graphics: Gigabyte GeForce GTX 670
PSU: Corsair TX550M 550W
Heatsink: Noctua NH-U12P-SE2 CPU Cooler NH-U12P
Case: Cooler Master CM Storm

I'm sure to get the help I'll need you'll want crash logs and such as well but I'm not really sure how or where to find those, if you detail them I'll try and get them for you.
Thanks for reading.

[Nixaler]

Sounds like to me if you had the heatsink on there and then took it off and put it back on, you will need to reapply thermal paste. I am almost 100% that is why his cpu is overheating and crashing. Someone please correct me if I'm wrong.

[Imnick]

This does sound likely to be the problem, which may be a pain to remedy as that heatsink is such an annoying thing to take off to reapply the paste
I'll just have to hope his original crashing issue was either a driver problem or a loose cable that we already fixed...

[yurano]

This does sound likely to be the problem, which may be a pain to remedy as that heatsink is such an annoying thing to take off to reapply the paste
I'll just have to hope his original crashing issue was either a driver problem or a loose cable that we already fixed...

Well, the heatsink reapplication takes 30 minutes if you're meticulous. If you want to rush it, you can definitely complete it in 15 minutes. Just make sure you use a solvent that can dissolve organics. I read that isopropyl (rubbing alcohol) does a good job and it does. Not sure if it damages your CPU though.

[Nixaler]

Well, the heatsink reapplication takes 30 minutes if you're meticulous. If you want to rush it, you can definitely complete it in 15 minutes. Just make sure you use a solvent that can dissolve organics. I read that isopropyl (rubbing alcohol) does a good job and it does. Not sure if it damages your CPU though.

No damages, just be sure to use a cloth that wont leave any lint behind