Since yesterday evening I'm having some trouble with my pc. It randomly gets blank screens (under windows, when booting up or even in bios). It just goes dark, if music was running at that time its repeated in short loops, fans get quiet and go on back at max speed. No response to ctrl-alt-del, have to press the button on my case.

Tested the RAM this morning, no errors.
Can't test GFX it is PCIe and the old gfx i have is agp which the board hasn't.

I came to think that it's either the board or the hdd (Sata), but can't test that atm... no other hdd (Sata) here.
It seems to run fine on a IDE hdd (installing atm). The Sata hdds smart status is ok.

Any tips?

Telediagnosis is always difficult 

It could be a Power problem (what PSU you got?)
but
I bet it's the GFX card as you said you tested the ram already and
because you also get black screens while viewing the bios.
Can't be corrupt card drivers as you don't use them (they are not even loaded)
when you are only viewing the bios.

Of course it could be the M-board itself but this is very unusual.

The HHD doesn't really have to do anything when you are in the bios.
If the CPU was the problem then I would think you wouldn't get into the Bios at all.
and if it was an overheating problem then it usually didn't start when you are just in Bios

What GFX cards you got?
As I noticed (some customers had it and I also had this myself with the new GFX card)
that a lot of the new 8xxx cards having these problems (but it's not only with Nvidia based cards also Ati is having the same problems:).

Randomly black screens, sometimes white and black (see Richards problems) , freezing PC
(btw I also had this sound loop stuff whenever I heard music or played a game and the broken GFX cards made the PC "freeze" get stuck in the black screen.

And I sometimes wasn't able to even get into the bios as the screen was just black.
Removed the GFX card, plugged it back in and the I was able to see something again on the monitor.
But in the end nothing helped except changing the card.
Would be good if you can test another card in your PC
else it's just speculation

small update:

I came to think it's either my PSU, my sound card or the dvd burner.

In order to test the PSU I removed some stuff from my pc (dvd burner, soundcard and one ram module).

I did install some benchmarking stuff (everest and 3dmark 06) as I wanted to see if it's a) a heat problem, b) a memory problem c) a gfx problem as I think this stuff really stresses the hardware. Anyway after removing the stuff and after having a test run with both ram modules alone I came to think that it might be the psu no delivering enough power and causing the crashs.

Gonna by a new PSU tomorrow (the spare one I gt has even less power (350W) than the current one and post the results. Or are there any tools to test the psu?

Current PSU:
400W XILENCE, XP400

Current GFX:
ati 1950pro

It's crashing again... 

At no blank screens this time, just a whole freeze. I think its the HDD as it keeps crashing when I'm copying a lot of Data (Backup harhar). Also had 2 bluescreens last weekend...

Though it seems to work stable atm under protected mode (or whatever it Abgesicherter Modus is called in the english speeking countries).

It can be everything  of course can dirt or greasy stuff inside the plug cause errors
but you can try other things too: (running the PC all the day in safe mode isn't really an option)

Check if you have a pagefile.sys file (maybe it's on this HDD and can't be loaded properly that can cause "kernel stack ..." problems. It's easier to explain it in german I think.

Hast Du nen pagefile.sys file unter C: ?
Wenn die kaputt ist oder nicht richtig geladen werden kann dann können unter umständen sensitive Informationen über dein System nicht geladen werden

1.
Falls kein pagefile.sys da ist: File  erstellen (siehe unten) oder aber die bestehende verändern bzw. abschalten und später wieder neu machen.

So geht's
Virtuellen Arbeitsspeicher (pagefile ) zu deaktivieren:
Start -> Einstellungen -> Systemsteuerung -> System -> Erweitert -> Systemleistung (Einstellungen) -> Erweitert -> Virtueller Arbeitsspeicher (Ändern) deaktivieren oder auf 0 MB setzen.

Später wenn der Rechner problemlos läuft,  virtuellen Arbeitsspeicher wieder hoch setzten.

2.
Wenn das hilft wäre es gut ansonsten kann durchaus die Platte der Fehler sein.
Was ist das für ne HDD - geh mal (als zweites) auf die hersteller seite und such nach nem dianose progamm.

3.
Bios updates solllen bei dem Fehler schon mal wunder gewirkt haben

Edit:
4
Nur mal so nebenbei
bei XP mit SP3 kannst du auch im Geräte Manager bei den Festplatten den write cache per hand deaktivieren


Hope it helps

Ich haben eine guten tag 

Ich forstehen nicht deine schreiben, nur kleine wort..du hast eine problemen mit deine PC. Ich haben nicht eine problemen mit meine PC. Ich haben eine guten PC.

Ich wunste that deine problemen mit deine PC kannst be fixed..

Ok that was without a dictionary, and I dont know if anything at all gave you a clue.. 

Haben eine guten tag meine freunde..