'Never change a running system'
I went on a full-blown shopping spree and ordered a whole new case that included a new power supply, instead of just buying the power supply only.
Unfortunately, it wasn't just the power supply. The PC still didn't want to start up, the screen remained blank.
I suspected the graphics card, but after testing that on my flatmate's PC it turned out to be perfectly okay. So I turned my ever-angrier gaze to the mainboard, for which I had no possibilities to test for malfunctions. 'Ah well' I thought, 'I'll just take the 90-minutes trip to the store and get it exchanged.'

Intermezzo
...and then I realized I had forgotten to take my purse out of my mother's car before she left on Tuesday. So there I was, no purse meant no bank cards, no ID, no drivers' licence, no student ID and no student ticket for the local trains and busses; which in turn meant I was completely stuck if I didn't want to throw away money for public transportation - which I didn't.
A call home later, my mom says she'll put it in the mail right away. That was Wednesday. The package finally arrived ... today! Congrats to the German Postal Service for taking 3 days to deliver a regular sized package! You guys are the hit! I'm not being sarcastic either.
End intermezzo

After getting the purse today, I had time until 14:00 to get to the shop and get the mainboard exchanged for a new and working thing. Shouldn't have been a problem, really, even with the connecting trains and busses being kinda slow, I should've gotten there by 13:15.
Now, after having used and been dependant on the German train monopolists for years, I should've known better than to expect trains to depart and arrive on time. Yes, you guessed right, I was late at one point and missed the connecting bus. Yay me, and yay DB!
I should've called it a day then and there and went back home, but you're always smarter afterwards, right?!

So I went and got a taxi to get me to that store, which cost me an additional EUR 20,- ... but oh well, if it serves the goal of finally getting this piece from Hell put together and in a working state, I'll gladly sacrifice something to the Gods of capitalism and economy.
I arrived at the shop at 13:30 .. and it was packed with customers! After almost 40 minutes of waiting, my number finally came up and I explained the whole story to the techie in charge. He looked at me, at the board and said "well, I'm afraid it's very unlikely that the error is to be found on the board, but hang on, i'll patch it up". Said he and slapped it all together with an exact copy of the components I got for myself. And guess what?! It worked without any quirks, just like that. I was baffled.

...and then I realized what the meaning of this was: it was the CPU that was damaged. It couldn't be anything else now. And the CPU ... was lying back home on my desk.

The techie must've thought I was going to faint I guess, he had a look of real concern on his face when I dared to look at him again after taking a couple of slow breaths to calm down. All the time and annoyance for nothing! I'd have to come back on Monday again, do the whole odyssey again, just because I hadn't thought of the possibility that it wasn't the mainboard after all, but the CPU, the only other part that could be the fault. All I wanted was to let out a howl of fury; but it wasn't the techie's fault, nor anyone else's, so I just tried to get a smile in my face (I'm pretty sure the outcome was closer to a maniacal grin, like that of a crazed out junkie with a chainsaw) and with a heavy sigh I managed to say "ah well, I'll be back on Monday with the CPU then, I guess".

Leaving the tech part of the store, the idea to check the sales part for a second and just grab another CPU there and just send the broken one in and demand my money back. Of course that didn't work out, because by then, the sales part had been locked down already. So had all the computer stores downtown when I got there one and a half hours later.

So now here I am, having wasted an entire day for nothing. I can't even be arsed to clean up my room (which is still a complete mess) or go to meet SB and make a fine dinner before I have to leave for work.

And I'm feeling a bit schizophrenic for looking at myself from the outside and finding it amazing how a small piece of hardware can have such a demoralizing effect on a person.

Now I'll go and get something to drink from the store around the corner and I think I'll give myself a little heads-up by eating out at some fancy place with good food. I've wasted EUR 20,- on a taxi already today for nothing, so why shouldn't I get myself some nice food to feel better again for less than that...

What did I learn today?

1) Never change a running system
2) If there are 2 parts which can be faulty, you'll pick the wrong one and forget to bring the other
3) If at first you don't succeed, go home and try on another day
4) High-tech is crap

Ta-taa.