Good thinking. I completely missed that.
Also the POST card itself is probably powered by 3.3v or 5v but not both.
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The M571 uses the SIS 5571 main chip and the internal PCI bus controller feeds the PCI-ISA bridge and both PCI and ISA slots, if the PCI slots are bad then it is possible the ISA is bad too. I have a PM9100A and it is nearly identical to the M571. Following the traces it appears the sound and video (ht1869 and um8670) are directly connected to the 5571 chip but the PCI and ISA go to differant sections of the 5571. The way to verify if the PCI controller is working would be with any ISA card (sound, modem, video, network) because the ISA bridge is tied in after the bus controller. Yes I noted that trying a ISA video card was suggested but think any ISA card should be tested before trying any other things. It would at least let you know if the PCI bus and ISA Bridge are working. Just a thought.....

I want to apologize, that I couldn't reply any sooner. I was a quite busy recently.

First of, I want to Thank Roman555 and PCBONEZ, for your great informations! I agree, it's indeed really odd, that my POST card is working, but every other PCI device isn't. But what's even weirder, is, that Windows seems to detects my POST card and wants me to install drivers for it. Are there even drivers for a POST card? Anyways, I'm going to test (once again), if any other PCI devices gets detected. I will contact you, as soon as i'm done testing. Sadly, I don't know, which half of the address/data bus my POST card is using. However, I could upload a picture of my POST Card, should this be helpful.

Secondly, I want to Thank Horun, for your great informations as well.
I thought the Chipset of the M571 was the SIS 5598 (aka TX Pro II), correct me if i'm wrong.
I'm afraid, I don't have any ISA cards in the moment, but i'm planning, to get one soon.
Although, before I'm going to buy any ISA cards, I'd like to make sure one last time, whether my problem is, indeed, related with the PCI Bus, or if it's a compatibility issue with my graphics card.

Which graphics card were you attempting to use?

A POST card doesn't need drivers.
They do their job before Windows even starts.
I suspect Windows sees the chip and doesn't know what it is so defaults to asking for a driver.
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bjwil1991, the graphics card, i attempted to use is a ATI Rage XL (PCI). This model is using a custom-made PCB. It is very likely, that this card is using recycled parts, that came from old server equipment. It was listed as brand new on Ebay.
PCBONEZ, thank you, now I know it better.

Yeah, reman cards from recycled parts can cause issues. You can find a used one (a good quality one) cheap, I believe.

I'm going to ask my friend, whether he still has his old ATI Mach64 GT, if not, there are plenty of cheap PCI GPUs on ebay and similar market-places.

Looks like he has already sold it.
I found 3 different graphics cards on ebay.
A ATI Rage II+DVD, a Matrox MGA220 and a SIS 6326
Which one should I buy, or shouldn't I buy any of them?
Which PCI Graphics cards can you recommend me?

I would get the Matrox if it is 4Mb, that would be a Mystique card. Others may have their own preference but never liked older ATI stuff, have had compatibilty issues with them. A PCI card based on the S3 Trio64 or 64v+ is one of the most compatible old PCI ever made, all OS including early Linux supported it well.

Thanks Horun.
I'm going to buy the Matrox Mystique, or the S3 Trio64 card.
There are plenty of cheap S3 cards on Ebay, for really good prices.
However, I'm not going to buying these cards now, because I still have to finish some other projects first.
I'm going to inform you, when I bought the item(s).

It's been a while, since I last posted on this topic. I apologize for that.
I finally managed to get a different PCI Graphics Card. It's yet another ATI Card. A Mach64, to be a little bit more specific.
And guess what. It finally works now.
The ATi Rage Pro card was indeed incompatible with my system all along.
I want to say thank you, one last time, for anyone, who sacrificed their time to help me with this silly problem.
Without you, I would've never got that far. You guys are the best.

PC Cheaps at its finest. Glad you got it all sorted out, man.