I've just put a friend's mobo in a new case, along with the parts from my current PC (which all worked) but it won't stay on when I power it up. The PSU fan spins for a brief second, and then stops. I've tried reseting the CMOS (even changed the battery), and I've made sure there's a good thermal connection between the CPU and heat sink. I had to do some soldering in the case as the speaker and power LEDs cables had broken, but I swept it out bfore putting the mobo in to remove any stray solder. All the cables seem to be in their right homes, but it just won't go. :(

You should always bench test a board before installing in the case. CPU/HS/fan, 1 stick RAM, video card or onboard, monitor, Keyboard. No cables of any kind other than the leads from the PSU to motherboard and video card, if required. No drives are needed to POST. Clear the CMOS before firing up and once it starts, immediately enter the BIOS to reset values as needed. To fire the board use a flat screwdriver to temporarily short the power switch pins. If all goes well you can install the remaining RAM and try again. If all is OK install the board in the case and try it again. If still OK then you can connect the drives, case to board connections, etc. You may have case standoffs in the wrong place. Standoffs should be located under each screw hole and ONLY under them. Are you even sure your mongrel parts are compatible? Are you sure the processor is supported by the board with the BIOS version present? The power supply can be a problem if it is undersized. Wattage ratings are kind of meaningless with no name PSUs.

The bench test works. The PSU, mobo and CPU are all from the same machine- I'd swapped my parts out from my friend's a while back; the mobo was the only thing left to be done. There was one mount point with out a screw, but I don't think that's the issue as there are no tracks in that corner of the board. Looks like the problem was a snapped mount (they're the old clip style). I've replaced it and so far, so good. Slow and steady is the way forward from now on. Thanks for the help, OtheHill. ^_^

You are welcome.