DrIstvaan: Sigh... it just doesn't seem to want to work well. I've found I can very well squeeze in the new drive reasonably (I underestimated the space inside the house), I got a Molex->SATA/SATA converter, now it looks like the HDD itself is faulty. The machine hangs for a long while when autodetecting HDDs, then it goes on to load up Windows as usual (since I didn't have to take out the original, it can still boot up), but then it always reboots.
I'll try installing it in my PC to see how it works there and check if it's indeed the HDD's fault or there's something else.

DrIstvaan: ...

DrIstvaan: Thanks for trying to help, everyone.
@HampsterStyle
It's an ASUS M2E-MX motherboard. I know where to find its manual on-line, but thanks anyway. Also, this is not the first SATA drive we're adding to it.

@Jayded
In Boot order, I can only set in which order should the different devices be loaded (optical drive, then HDD, then floppy (yes, very likely we'll still use that, but anyway ;-))).

@Lexor
It is old, but not "ancient" (it was assembled at the end of 2007). It should be able to handle a 250GB HDD (given that there are already two or three /I can't say for sure if this is true for all/ HDDs which are at least that big).

cjrgreen: Certain second-generation (3.0) SATA disk drives have issues with certain first-generation (many older motherboards with VIA or SIS chipsets) SATA controllers. This sounds like one of those problems. That makes the exact make and model of your motherboard and the exact make and model of your disk drive important.

M2E-MX is not a retail ASUS model number. Did this motherboard come out of a proprietary computer (Dell, HP, etc.)? Or is it M2A-MX or M2N-MX?

And what is the make and model of the disk drive you are trying to attach?

DrIstvaan: Sorry, it's an M2N-MX. The HDD is a WD 250 GB one, but I'll check the details and get back to you.

cjrgreen: M2N-series motherboards should not be a problem with any generation of SATA drives. They have an nVidia chipset with solid SATA 3.0 support.

I'm wondering whether the power or data connection to the drive is good, or the drive itself is good. A SATA disk that persistently won't detect at POST may well be defective. Or even the power adapter may be defective.

The hard disk market has been chaotic for months, and especially in countries outside of mainstream distribution, such as Hungary, there may be questionable drives being sold.