HTTP Error 401.1 - Unauthorized: Access is denied due to invalid c

I am getting this error, when trying to access any of the sites hosted on our
IIS server.....

HTTP Error 401.1 - Unauthorized: Access is denied due to invalid credentials.
Internet Information Services (IIS)

I created a new site that I can fine, I have attempted to compare the
security / permissions on the new site to all the older ones and everything
looks the same...

Please Please Please help....

Thanks