I have P4 Prescott 3.0 GHz BOX edition. The cooler is from the box. When I turn on the PC the temperature is 54-58ºC but when I start an application that uses 100% of the procesor the temperature jumps to 142ºC. I only run that applicaton for a few seconds.
How it that possible?

Do you have thermal paste on the H/S or the pad intel puts on it? Is the h/s sitting flush with the cpu?Also make sure the fan is working.Try grabbing the whole heat sink and turn it back and fourth not hard just enough so that if its no t flush it might seat it .

Of course. I am not that stupid.

Maybe a bad sensor? Or is it really that hot... try touching the heatsink when it shows that temp.

I didnt mean it like that.what i meant was paste or the stock pad.It could be a bad sensor also thast a definite possiblity.

It`s hot but i don`t think that is 140ºC .

What is your voltage set at?And what mobo are u using?abits tend to read high but not that high. Dam thats 287 degree's in F.No way that Cpu is that hot the cpu has a built in thermal protection that would not let it reach that high of a temp so it has to be a sensor.

Voltages are
+1.5 Vin 1.480 V
VCCP 1.247 V
+3.3 Vin 3.396 V
+5 Vin 5.263 V
+12 Vin 12.250 V

I don`t understand what this means.

Mainboard is Intel Socet 775, D915PSYL, mATX

Its just the amount of voltage you system is getting or needs.But all your voltages look fine so its got to be a sensor that went crazy .(they do that sometimes) Are you using the intel control panel to read these temps?I would go back and try it again and see if it still does this.I really think it was just a bad command to a sensor or something.It might have been a one time thing.

I tried reading the CPU temperature from the bios and it is 140-142ºC.

Either something is extreemly wrong or a bad sensor. Did you flash the bios? Had to do it with this one to read the correct temps.

I did flash the bios. I`ve got it from intel`s homepage

That is wrong for sure try looking on intel's webpage to see if its a gliche or something that can be fixed im looking for you now.Some time they know about things like this and the have fixes for them.Also go to this link and down load the everst home edition it will read all your temps and anything else about your computer
http://www.lavalys.com/products/download.php?pid=1&lang=en&pageid=3

http://www.intel.com/

There is a bios update but i already have that one.

Do you remember which versions of the bios it was because they might have an update out by now.I was just reading about them fixing some with cooling problems in one of the bios.

I've just downloaded the EVEREST HOME EDITION and it shows the temperature of my cpu- it's 47 C.
i think there is something wrong with the sensor because it is unknown (SMBus 2Eh)

Does anyone know is it ok to have processor temperature around 63 celsius degrees when running ''heavy'' applications

Sure with the pressy thats normal running heavy ap's.Heck my northwood hits 57c sometime not often but sometimes..A couple things you coluld do to help temps some add a couple fans.. get a zalmen 7000 copper H/S these things are great.And or take off the stock thermal pad intel supplied and Add some Artic silver 5 and lap the heat sink and the cpu .Get some sand paper and sand them smooth and flat then get some 600 grit sand paper and sand it til its nice a shiny. you can buy lapping kits they are cheap .But all you need is sand paper fine grit with them couple of things you could drop your temp 5 -7c easy and with the zalmen heat sink high 40's should be within reach.

http://www.frozencpu.com/cgi-bin/frozencpu/cpu-zal-11.html
Try this link this H/S will do wonder for your temps