Well perhaps not quite so dramatic but my PC is now working again. I tried various thing this afternoon, all the usuals, re-seating cards, disconnecting drives, different graphics card and nothing would bring my machine back to life.
As a last ditch effort I removed the BIOS battery from the motherboard just while I was thinking of something else to try. Out of curiosity I powered the machine back on when I re-inserted the battery and it booted up!
I took a while for Windows XP to sort itself out and report all it's errors back to Microsoft (what does that do? does anyone take any notice?) and I'm now typing this blog entry on my PC again.
It's good to have it back :-)