My code-behind has an OnClick handler that ends with a "Server.Transfer("AnotherPage.aspx"). If I get to the Default.aspx page by entering the full URL into the browser, (i.e. http://www.MySite.com/Default.aspx) it works. But if I go there via the default page feature (i.e. http://www.MySite.com/ without the "Default.aspx"), it doesn't. In this latter case, my page posts back to itself. This is not a problem when I run it within Visual Studio - only when I publish & install it on Winhost. The same thing happens if I replace "Server.Transfer" with "Response.Redirect".
What is your default document page? It sounds like your account is configured to start up a page other than 'Default.aspx'.