No idea if this will be of use but posting anyways - just in case it might help someone else.

Late afternoon today - attempted to 'wake up' my internal CARD reader - now and again it is not showing in explorer and usually a disconnect of USB cable and re connect encourages WIN to ''see'' it - and the four ''DRIVES''.

This time - BSOD!!!!  Horrors ... and a message saying approx ..... "Parity error/memory - consult your hardware vendor"  Yikes!!!

Shut off - powered off totally - let rest then try again - monitors blank with no signal and CPU fan goes to high speed - and that's it!  Zilch.  Repeat - same.  Sinking feeling in belly!

I could get a recovery CD to read and booted with that but did not feel like any major attempts at that stage to try any rescue in case matters went worse.

Spent a while updating data on spare machine with external drive, ready to have to switch work to that one.  Just one idea to try with main machine - let's check mem sticks - long shot.

Removed both 1Gb sticks - swapped them - so #1 stick became #2 and VICE versa - refitted with care for full seating ........ and heavens be praised after powering up again - booted!!  Quick spell in safe mode - all looked OK - then boot normal to usual win ... success.

So - message here might be that suggestions made in posts in past about removal and reseating of boards, mem etc - can just help.  In my case I suspect (maybe) the mem stick reversal somehow achieved a reinitialization effect ...... main thing is - back up and running - right now anyways.

Any ideas from anyone else would be interesting if any actual explanation.