Well, today my sis turned off the pc in the normal way (she says anyway), and when I come to turn it on, the whole thing starts up at normal with the usual working-sound-thing, which usually stops at some point - only this time it dont. The sound gets higher and higher, it sounds like the fan, but Im not sure. It seriously sounds like the PC is gonna explode or something, the sound gets unbelivably high, and just keeps going - so I turn off the power immediatly. Then, it happens next time, and again and again. So whats going on?

Infomation:
PC is dekstop computer with separate screen, keyboard, etc (obv..)
All the wires from /to the pc go into one transformator box, which is either on/off by a red knot, meaning total power to whole pc system or no power at all - and as soon as I turn this on, the pc starts by itself -> which it normally dont, you should turn the "Power" button on both the screen and the pc driver itself.
Its 4-5 years old
This has never happened before
I tried turning on and off the screen, pcbox itself and all else several times, but the "exploding" sound just gets louder and louder - pc doesnt respond to the power button
The screen is all black, it doesnt get to the windows loading screen or anything
I have not tried just leaving it to sort it out - because I am not sure if there is some lesser problem which can turn into a really major problem if I just leave it
The "exploding" sound as I described sounds like a fan, which is a normal sound to some extent, but it usually stops increasing quite early in the startup process.

If anyone have any tips/link I would greatly appreciate it
(internet works fine so can check on here from laptop)

I can add system specs later I guess, but just too upset to find them at the moment (I cant remember them), but if anyone have any usual tips (even though its lacking useful information, I know..) All useful tips and hints are appreciated, and I can describe to you as much as I can see/hear, but if you need system specs I will have to search the whole appartment as I have no clue where they are
(Obviously cant get into the pc and check the specs from there)

Could also add that when power for the drive is on it starts by itself with these sounds,
The power icon on the drive is Yellow,
The monitor does not respond to the pc (i.e it says "Check signal cable" etc and such stuff that it says when monitor is on without the pc drive

Hi - Welcome to PCHF - the talented team will endeavour to help.....

but what can I say.................

It's either the PSU, motherboard, VGA card or any other component.

How capable are you at swapping bits in the PC? If compotent try another PSU first.

Sorry - no magic wand!

If I say I have never done it before/know anything about it?

Well, do you think it would do harm to just leave the pc and see if it eventually works out - or are these sounds a sign of danger, and I should avoid turning it on at all?

Assume it's beyond economic repair.

Now whatever you do doesn't matter so long as you don't endanger yourself or others.

However is there any critical data on the hard drive?

All critical files are on an external harddrive - (just bought that 2 months ago, lucky eyh).

2 guys have suggested it is likely to be;
The videocard, since the monitor doesnt respond but keyboard does,
Dust bugging around inside the pc, as it is apparently a common problem, causing the fan / videocard to work improperly - does this sound likely?

And also, the fact that the PC turns on automatic as soon as the power to the harddrive is in - isnt this a bit weird? (I dont mean as soon as I push the power-button, it is litterally as soon as I plug it in the wall it starts by itself).

Update:
Left the pc to try to sort out for 2-3 mins, nothing happens.
The monitor doesnt respond and the power-icon is Yellow, so I am not sure if the pc is really on at all..it still doesnt not respond to power-button, or F8 and such