Hello Spanners,
Have to agree about Nortons, bin it quick.
I myself had similar problems, seemingly unexplainable, tried allsorts, swapped RAM, cleaned windows, re formatted to no avail. Eventually changed CPU cooling fan (on a guess) and solved everything. I don't know why or how I arrived there as the old one looked and sounded perfect, but obviously wasn't doing the business. The system was less than 2 yrs old.
Hope this helps.
JLM