Hi cossack,
I use NSW2001 too and have come across the same "Unable to write to drive C". However, it seems to happen whenever I or the system is "doing something" on C
. If I leave well alone and let speedisk run its course, there aren't any problems. The habit now is to start it running after a boot and leave it alone until finished. No problem.
I wouldn't bin it if it's only happened the once: give it another go. Since it's software it won't harm the disk physically by trying again. Also, if MS defrag didn't fix it, it's even less likely to be a problem with Norton.
FYI the same thing happens occasionally when "image" is running. It doesn't like other activity on the disk, which gets in the way of the snapshot being taken.
Good luck either way.