I don't think the symptoms match a software problem! The fact that id can freeze before I even log-on makes me suspicios for a start. Also, there is nothing I can think of non-standard (i.e. not part of Win7) on there yet. I only reinstalled Win7 the other week, hoping that would cure it. Finally I tried another fresh install on a different disk and that froze too.
I'm going to run the memory tests again (overnight) and think about buying a new motherboard in the morning:-(
Would be good to know if there is anything else to rule out or of any conclusive tests I could try first!
Thanks for the suggestions so far.