OK, I've just stumbled across something that I thought you might find interesting:
I hadn't thought of checking for other users since (apart from guest and admin) I'm the only user created .. or so I thought. Well, I've just checked User Accounts and suddenly there's a new user there "ASP.NET Machine A..."
It's limited (not administrator) and is password protected though what this is I have no idea.
Now the other day I installed the .NET framework (all 3 versions 1.1, 2 and 3) because I wanted to install a program that needed it (ABRViewer; a utility for viewing Photoshop brushes if anyones interested - and very good too - but I digress).
Now the ASP.NET user does not show up on the Welcome login screen, but I guess it does explain why the screen has suddenly started appearing. I'm guessing the ABRViewer has to somehow declare itself as a user. Anyway, thought you might be interested to know about this and thanks for the Tweak UI solution btw

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