NetworkError: 404 Not Found IIS Windows 2008 R2

I have dedicated windows 2008 R2 server where I have IIS website in html website. Every thing is working fine but on some of the pages when I go to firebug I can see that some css and jQuery files are showing NetworkError: 404 not found

But when I go to the directory on the server physically files are there then why it is showing not found !!!

can somebody tell me what can be the issue!!