I was given a PC for repair with one major problem: it does not turn on at all, not even fans spin.
The reason of failure was likely a bad PSU. The owner told me that when they tried to turn it on, the PSU went boom.
I have opened the PSU (Branded “ISO”, a Channel Well budget series. They are OK PSUs except for capacitors) and found 2 bad capacitors (the infamous F*u branded capacitors). The PSU still sort-of worked (5Vsby was 5.24V) but it whistled badly when powered up (didn’t try jump-starting it).

I have installed by bench supply (a 350W FSP) and tried to power the system on, it was dead.
Then I started examining other components. Removed the processor and installed it back, checked RAM.
Then, out of curiosity, tried to turn it on again and it worked ! Booted to XP without problems.
I thought “OK, it looks like only the PSU bit the dust, the itself system is fine” and bought a new PSU (400W FSP).

Installed this PSU to the system and tried turning the system on and… it wad dead. WTF ?!!?!??
Again, checked cables, components, tried clearing CMOS, tried my bench PSU again. Nothing. The system was stone-dead.
(I did manage to get the system to at least try to turn on once, but it turned itself off after a few seconds).
Well, it seems that the old PSU did damage something when it exploded…

The happy part for me is that the owner decided to give away the system to me (for parts) at the cost of recovering information from the hard drive.

System specs (from memory, too lazy to look up):
MB: Gigabyte, Socket AM2
CPU: Some AMD Athlon 64 (single core)
RAM: 2x512MB (1GB)
VGA: GeForce 7400 or 7600 PCI-E
HDD: 250GB Samsung SATA (in perfect condition)
Also, some optical drive and a generic black mid-tower case.

HDD went to my NAS (a hacked F7D3402, mentioned in the previous post, had to do some mods so that the HDDs would mount properly when connected to USB Hub, but thats a different story), other parts are still in the case.

Lesson learnt: Avoid cheap PSUs. Always remember to check your PSU from time to time. If it whistles or makes any unusual sounds (eg. whine), you should stop using it, unless you know the PSU should behave like that (my new Corsair TX850 V2 did whistle on standby when it was new, but stopped whistling after some time).
A bad PSU can take the rest of your system with it when it dies !

P.S.: I had another system brought to me for an upgrade (a LGA775 based PC with 2.8Ghz Celeron D and 512MB RAM with XP on it, upgaded to a 3.4 Ghz P4, 1GB RAM and Windows 7)
Before upgrading, I decided to check the PSU, as it looked very much like one of those generic chinese cheapies (even though it had “Q-tec, The Netherlands” on the label), and, surely enough, found a bad capacitor in it (again, one of these damn F*u caps). Replaced this PSU with a 400W FSP (mentioned earlier).

Enough for now…