I have just taken the cover off my PC and cleaned out all the dust

I took the heatsink off the processor to clean it and re attached it

now my PC keeps shutting down and is saying it is overheating

the fan is on constantly

what have i done?

is it dead?

You should reseat the CPU HSF and apply a fresh coat of thermal paste onto the CPU, first removing any of the old, dried out paste with isopropyl alcohol (rubbing alcohol). A very light coat of paste will be enough.

The fan-heatsink assembly must be seated with properly applied thermal compound.
"Arctic Silver" is a popular brand. You can download instructions from their website here; http://www.arcticsilver.com/instructions.htm

Please note; none of the thermal compounds are "wash and wear" so to speak. they must be cleaned off and reapplied, each and every time the heatsink is removed for any reason

Please do not attempt to keep firing up your machine until you have resolved the problem. Even with thermal safeguards in place, it's still possible to do significant damage or at very least shorten the life of your CPU.

It's also possible that the fan itself is dying, you may need to replace it or buy a better one. As for thermal compound, I highly recommend Tuniq TX-2 or the new OCZ FreeZe stuff... better than AC5, especially because both compounds do not conduct electricity.

I have put some new thermal paste on and it seems a bit better

Maybe I should replace the fan and heatsink

Do all fan/heatsink combos fit or do I have to look for a specific one?

If it is still running hotter than 'normal', you may have a misalligned heatsink or a mobo pop-in plug that isn't catching.

Depends on the cpu type which you didn't specify.

My CPU is listed as Intel Pentium 4 3.40GHz (2 CPUs)

If you have a mobo manual the better ones describe how to properly attach the socket 775 heatsink.

If you don't have the manual or cannot get good instructions from it when you get one via download, download one from Asus (any socket 775 manual, Asus P5GD1 for example) and follow the section on attaching the heatsink - they have good instructions and good pics.

I looked at the manual and that is how I re-fitted it

The fan has been coming on way too much lately anyway, which is why I took the case off to clean everything out hoping that it would help

Is it worth looking for another fan/heatsink?

Response to "fan running more".

Have any high demand new programs been installed recently which would cause this? After all, "Crysis" would make your machine run hotter than "Spider Solitare".

If the machine is running without shutting down, you might try installing a program such as "Speedfan" or "Everest" to monitor what the system temperatures actually are.

If this is an Intel motherboard, they have a monitoring program called "Intel Desktop Utilities" available for download.

installed speedfan and my CPU temp is about 62 just running firefox with about 8 tabs open

I'm interpreting this as a "Pentium D" (dual core). They're notoriously hot running CPUs, but still, Firefox with 7 tabs open is hardly even a minor disturbance in the force. Run "Spybot SD1.6 and see what your temperatures say. This program will push the CPU usage to 100%. BTW, what does task manager say the CPU usage is?
You might check what the actual fan RPMs are. You also might to just let the box sit at idle to see what that temp reading is. If you're sure that you've installed the HSF correctly, you might want to try again anyway, just to make double super sure. A little dirt on the CPU, too much or too little paste, a not quite seated pin could make all the difference. See if this page is any help; http://www.intel.com/cd/channel/rese...ech/375680.htm

I don't think so. The moniker may refer to either the Northwood, Prescott or the Cedar Mill editions of the Pentium 4 HT series CPUs. So the idle temp may or may not be normal.

But yes, I'd blame it on an improperly seated HSF or too much TIM.

The fan has been coming on for a good few months as soon as I try and do anything on the PC

I thought that it must be full of dust so cleaned it yesterday and removed the heatsink

I have refitted it twice following all the instructions but it is still acting the same, it hasn't shut itself down since yesterday, but somethings not right

I will try the spybot and also try leaving it idling for a while and see if it makes any difference to temp

I forgot the HT CPUs can show up as dual processors, my bad sorry.!