Thanks again leo. As it turned out, the Auto-size columns option had been checked (this must be the default setting, since I hadn't touched it). However, upgrading from 10.0.1.0 to 10.0.2.0 seems to have cured the problem.
I notice Thraug's thread New filename, visually truncated ([url]New filename, visually truncated]) is a very similar problem, also you have posted an excellent tutorial about folder options here:
[url]Folder Formats: Quick Guide]