for a three year old system, I doubt it's the harddrive, but cannot rule it out. I'm more swayed towards the CMOS battery. Have you made sure it's in right, or can you pop it out and use a battery tester on it?
If the battery goes, so do the settings in the CMOS. Each start up the computer has means you'd have to set the hard drive settings from scratch. While the power is on, however, the CMOS does not use the battery, saving it's life a bit.
Try suggesting that to Evesham. If this is not the fault, then I would be more inclined to think hard drive failure. A loose wire you can't get to, or a damaged component. If the computer hasn't been moved or hurt in three years, this can only be caused by either heat or old age. Or dust.
Anything can go gradually or just switch off. Gradually would be power fading or errors start to be made. What you're experiencing seems to be a complete lack of usage.
I might be rambling and missing the point. If anyone can spot anything worthwhile in this post, please point it out. Pronto. :)