Thanks for the suggestions - I uninstalled/reinstalled and checked for the Esellerateengine.dll where you suggested. Everything looks good, but the problem remains.
Then you have some issues with permissions settings, see below.
The only thing not standard about my installation that I can think of is that FSX is installed to a separate drive, D:\FSX. Is that supported by your installer?
Of course it is. We obviously don't make *any* assumption about were FSX is installed, but clearly ask Windows were it is. So, it will work regardless of the drive.
Under Vista (no idea on XP SP3, perhaps some of the Vista feature were adopted in XP as well) there *might* be a permission problem by installing something on the root folder, in some cases. Check that your user account have full read/write permission on the FSX folder and subfolders.
However, the only repercussion of wrong permissions on the FSX folder would be that AFTER purchasing, the .REG file wouldn't be written, but this would be just a minor annoyance, and it would't affect the purchase itself.
Also, I'm assuming your user account HAS admin rights, otherwise something might not work.
Have you tried running FSX as administrator ?
Also, have you tried turning off ALL security software, especially firewalls (the Windows firewall AND any other software you might have installed) before running FSX ?