I have a 4 year old self built PC that suddenly started shutting down, not rebooting then finally stopped working all together.  When powered on, HD lights would light, DVD drive would start, fans would run...but after 5-15 seconds it would shut down.  Tried many times and no success starting.  Took case apart and found the CPU heatsink/fan loose.  I have not tried to lock it back down and reboot yet (see below).
Motherboard is a LGA 775 based Asus P5K Deluxe and CPU is a Intel E6750 2.66 gHz Core 2 Duo with stock heatsink/fan assembly.   ETA: I forgot to mention that this PC sits in a kitchen office, under a corner desk and I'm wondering if the airflow is part of the issue.
I am assuming that the thermal paste is compromised so that even to test the current CPU/heatsink/fan combo I need to repaste it and then mess around trying to get it properly seated and locked.  I remember when I built it it took many tries to get it to lock in place.    Any advice is much appreciated.  Why would it loosen like that?  Heat?
This situation got me rethinking of the CPU upgrade I've thought of a few times.  I'd like to breath some new life into the PC by going to a quad core and buy myself another year or two to build a new Sandy lake based main home pc.  Probably move this to the kids room.  Did a little research and see a few LGA 775 Core 2 Duo quad cores still out there.  Prices blew me away for such an older processor, but I found a Q8400 for $169 at newegg.  Seems like a decent performance/price balance.   If I chose this route should I stick with the stock heatsink/fan or get something different?  Suggestions?
Update:  Reseated the cpu heatsink/fan then tried to reboot.  Dead.  Nothing.  The power led on the motherboard doesn't lite up at all.  Now thinking its the power supply.  Too late to try to find a store tonight and none open tomorrow, might just buy a new one via newegg, get the cpu and be done with it.  I hope.  Anything else I should be looking at?  Nothing happens on the power supply, no hum, vibration or fan movement.
 

If you're comfortable opening up devices where there is 120V electricity, open up the power supply.  You have to break the warranty seal, and one of the screws will be under tape or a sticker.  I'll bet that you find at least one capacitor bulged or blown.

Yeah, PSU'd be my first guess. CPU and mobo could have gone together; I for one wouldn't risk a new CPU on a potentially bad mobo.



Get a new PSU, CPU, and mobo.

I'm fine with breaking it open, I'm sure its dead.  Whats the risk in opening it up?  Nothing to repair, right?  just to know for sure?





 

Yeah, I was afraid of that.  Not really looking to rebuild 80% of PC, esp with the RAID 1 config I have I'm guessing a new mobo would mean I'd lose the data on the existing drives?  I'm going to replace the PSU first, get this one up and running, then consider the new CPU, mobo, etc.





 

The risk is in the high-voltage capacitors after a diode bridge, they can be charged to about 170V.  It's highly unlikely that they would be charged if the unit has been unplugged for any significant amount of time (bleed resistors are virtually always used), but you never know, especially with cheap ones.

You could, potentially, try to repair it by replacing any defective capacitors present.  Sometimes that works, sometimes other components have been damaged when the caps blew, the unit lost regulation, and voltages went all over the place.  But... I'd probably just buy a new, higher quality PSU to replace it.

5-15 seconds is not enough time for your CPU to reach it's high thermal barrier, esp. since it takes 30+ seconds to even get into windows. Stock thermal paste.

What brand PSU so the community can weigh future buying?

Enermax is what I dig.  Out of the couple hundred I've put into service over the past six or seven years, I've only had one die on me.  And they're not grossly over-speced, like many supplies.  I have a 305W Enermax which, under load, pulls 270-290 watts from the wall.  And it's been in use for about six years now.

Antec 500w, came with the Sonota III case.  Just purchased a Antec BP550 500w model from newegg.com for $60.



 

Nearly all of the Antec PSUs that I've used have taken a dump within two years.

Installed new PSU, booted and its working well.  Seems to be a much heavier duty PSU, big cooling fan on bottom of unit (inside of case) and larger external fan also.  Good modular power cords of all types.  3 yr warranty.