Memtest & scandisk are ok. The installs are on seperate hdd's. The box is an Antec fusion black with 1tb across 2 hdd's. The memory and all other peripherals are well seated & the case is clean and free of dust. I agree that hardware is probably the thing.
I have just updated the sound card and gpu drivers via the mfr's websites rather than via device manager, so we'll see what comes of that. The psu on these is a chunky 430 watt & seems ok, I reckon for now I can discount it.
Thanks for the input chaps, I appreciate it. As I say it's a well built, protected and maintained box, so a puzzle to me.