Not to sound patronizing, but have you checked that the IIS anon user has permission to write to the file?

Nothing patronizing about that, it's not something I'd though about nor to my recollection, done anything about when I first installed IIS and got it all working.

However...

It's all happening on the one PC, this one. So IIS and the website data files are all on this PC albeit on a different logical drive. The username is the same as before, mine, as I'm the only user.

At the Web Sites branch of the IIS tree and going to Properties / Directory Security / Edit Anonymous Access it refers to a user name IUSR_computername with a password of something or other. Not sure whether that's what you're referring to. But I'm almost certain I didn't change anything in that area before the rebuild.

Sort of what I'm talking about.

The NTFS file permissions on the files that are being written too by IIS need to allow the IIS user to write to them.

This can be acheived either by giving the "everyone" group the necessary permissions (not a good idea in a production environment, but not so bad in a test environment) or by specifically granting the permissions to the IUSR_computername user.

I'm not saying this is the issue. But it does warrant checking.

If you have the folders on a second drive, and have copied them manually I would suspect that the "everyone" group already has permission, unless specifically set not to.

However, you have stated that you reinstalled XP, in which case the NTFS permissions may still be referencing the UUIDs of the users and groups of the old installation, and so may not work as expected.

I couldn't see how to change the group permissions, Windows Explorer didn't give me the choice.

Howsoever, as the file was only a 4 byte test file with a count in it I created an appropriate text file on the C: drive, copied to the test folder, and bingo it worked! And the approach also worked for the other two test sites I run under IIS.

As it is purely a very local test environment and is never exposed to web access, permissions are less relevant than they would be if it were web visible.

Many thanks for guiding me to a solution.

No problem, glad I could be of service.

For reference, to change file permissions you first need to turn off simple file sharing (which works in exactly the same way as none simple file sharing under the hood, it just doesn't let you tune the permissions properly) and then in the properties box will be a security tab. This is where to edit permissions.

Ah, that option at the end of the Folder Options View list, not played with that - yet

One word of warning, when messing with NTFS permissions NEVER set a deny permission on the "everyone" group, and only set deny permissions on groups if you are sure that you will be able to unset them (Do not add the local admin account to any effected groups, for example)

Yes, I've had experience of settings various permissions on Windows Servers, always good fun. And on mainframe systems long before that. Care is indeed needed.