Html displays different whether i open it from the filesystem or from localhost

 Hello,
I am having a very weird problem regarding how a menu is displayed. To show it i prepared an html and the required css file, along with some images needed. They are inside a test folder in c:\inetpub\wwwroot\ (Windows VIsta 64 bits).
 If I open the html page from localhost, i receive the menu rendered as this:

But if I open it directly from the filesystem, the menu shows like this:

Does anyone know why this could happen, and how i could solve it ( ...