Hi Peterweb,
THanks for the thought. I've given the chkdsk approach a go, but it doesn't look to me as if it's likely to be the problem - at least, the final log says (amongst other things):
"Windows has checked the file system and found no problems. 968377343 KB total disk space. 729577068 KB in 373192 files. 213636 KB in 54630 indexes. 0 KB in bad sectors. 580779 KB in use by the system. 65536 KB occupied by the log file. 238005860 KB available on disk. 4096 bytes in each allocation unit. 242094335 total allocation units on disk. 59501465 allocation units available on disk."
In case it's of interest, the status with Norton is that the 2nd line support guy, having spent two days attempting to trace the problem, has taken a log of all his efforts to give to their engineers to see if they can come up with something. I'll give a synopsis here if they find anything that turns out to be potentially useful...