Have you tried without using the user credentials.. Just comment the userid pwd and try.. the following code works for me..
System.Management.ManagementScope ms1 = new System.Management.ManagementScope();
using (System.Management.ManagementClass exportedShares = new System.Management.ManagementClass(ms1, new ManagementPath("\\\\machinename\\root\\cimv2:Win32_Share"), null))
{
System.Management.ManagementObjectCollection shares = exportedShares.GetInstances();
foreach (System.Management.ManagementObject share in shares)
{
Response.Write("Name: " + share["Name"].ToString());
}
}