Day three without a PC so I decided to take it to David to see if he could fix it.
Result - The Seagate 500Gb Sata harddrive is kaput.
It's the second drive to die in less than a year! This one just gave up without a real warning. No virus, no grinding noises.
Luckily I back up my contents so it's all safeishly (new word) saved on another hard drive.
This is a shot of the open case, positioned so I see the fans that keep the thing cool and ensure the drive doesn't overheat. Thanks fans! It's snapped on the iphone in hipstamatic mode.