I'm not sure if the correct answer has been given, I kinda got tired of reading through the private argument among the posts...
Anyway, whether its MS running IIS or Linux running Apache the default page is determined by the web hosting company, so only they can really answer that.
However, what is commonly done is that they will have a hierarchy of defaults, as an example:
index.html
index.asp
home.html
home.asp
meaning that in this example if you had an index.html and an index.asp, the index.html would be displayed by default because its higher in the hierarchy....
Regards,
John