Thanks NAC Inc - all noted.
My PC is now sorted. The new RAM is the most likely cause, though the supplier did the whole job, including Win XP installation. He said that there were no unusual problems, so I'm none the wiser as to any little tricks.
The PC manufacturer, Carrera SSC showed no interest whatsoever -- they were aware that I had a problem with one of their PC's still under warranty, but not a single phone back (I have caller ID and an ansaphone - anyone even dials my number and I know about it)) and not a single response to email.
I mainly wanted to thank you all for all the ideas and for all of your time and effort - much appreciated !! 