Let's exclude the obvious:
Did you install any of the official patches on top of the vanilla install? Maybe you confused patch order.
Have you installed any of the mods?
If I get this right you only installed the game from the CD (that's version 4.07), it installed fine, but now it won't work?
Did you try installing the game on any other computers to verify it's not a faulty CD (scratched inbetween since your first installation)?
Other than that - running out of ideas...
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