All was going well and good, but for the fact it was turning itself off, generally within around 10 minutes. He rang me up several times, I gave him basic diagnostics, he got nowhere. I went over to check it out. Tested the power supply in another PC, worked fine. Concluded it was something to do with the motherboard, either software or hardware wise. Had a thought, and asked him "Did you take the plastic off the bottom, peeling off from the heatsink paste?" "Er, dunno" came the reply.
Hopped into the BIOS, and checked the temperature. Watched it tick up. 32. 34. 36. Right up to around 68 when it cut out. Decided the processor obviously wasn't getting cooled, so I unclipped the CPU heatsink and fan. Lifted it up, and sure enough, the heatsink paste was all intact. Underneath the huge plastic cover, which covered the whole of the bottom of the heatsink. Which was beginning to melt. Somehow, he'd had managed to leave the original plastic cover on the bottom of the fan/heatsink underneath the heatsink itself, locking it in between the heatsink and processor. So the CPU was in direct contact with... plastic. Which had begun to get deformed and start to melt. Oops. This wouldn't have been anywhere near as bad, if it hadn't been blatantly obvious, at least 1cm deep on the bottom of the heatsink, and the fact the silicon heatsink paste was underneath it, very well protected from any actual heat.
Suffice to say that it cheered me up no end that evening, and everyone who heard pointed and laughed. Including me. Repeatedly.
Funnily enough, after removal of the plastic, it ran at a nice cool 25oC. Shiny.
Lesson of the day: remove the plastic cover off the heatsink BEFORE installing it to a PC. It tends to run much much much better. And not hot enough to boil your coffee.