Need help with Forms authentication

Hi, I was trying to set up the Forms authentication via Active Directory on my test website. I followed all the instructions inside here: http://msdn.microsoft.com/en-us/library/ms998360.aspx
Unfortunately, when I try to login, I get this error message:

 Error: You were not connected
because a duplicate name exists on the network. Go to System in Control
Panel to change the computer name and try again.
 (C:\Inetpub\wwwroot\wss\VirtualDirectories\testSite80\web.config line 120)
 
Here is my membership details in Web.config
<membership defaultProvider="MyADMembershipProvider">
      <providers>
         <add
         name="MyADMembershipProvider"
         type="System.Web.Sec ...