OK I'm back!
Just did another unattended install and it works to some extent. If you use the same unattended.txt file, it tries to create exactly the same folders. As there are already existing administrator folders (on a profiles partition) XP adds ".WINDOWS" or ".COMPUTERNAME" to the end of the folder to make them the new profiles.
Basically I've ended up with the following profiles:
Administrator
All Users
Administrator.COMPUTERNAME
All Users.WINDOWS
In my opinion, this is not a bad thing though: if u keep an up-to-date registry bak-up, it would be easy to merge while in safemode so XP points back to the original user profiles. You could then delete the newly created profiles (the ones with the .WINDOWS or .COMPUTERNAME), and hey presto you're up and running with a fresh install and ALL your files and proggies intact!
Furthermore, I suppose if you went into safe mode and amended the names B4 u reinstall the new profile folders wouldn't be created by XP in the first instance.
I haven't tried the regfile merge thing yet, as I had to reinstall the whole shebang the other night. This has what's got me into this unattended install mularky, and so far I can only say I'm made up with it!
Comments/opinions welcome.