public static PrincipalContext GetPrincipalContext()
    {
        return new PrincipalContext(ContextType.Domain);
    }

    public static UserPrincipal GetUser(string sUserName)
    {
        //Save documents to library
        PrincipalContext oPrincipalContext = GetPrincipalContext();
        UserPrincipal oUserPrincipal =
           UserPrincipal.FindByIdentity(oPrincipalContext, sUserName);
        return oUserPrincipal;
    }

at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
 at System.DirectoryServices.DirectoryEntry.Bind()
 at System.DirectoryServices.DirectoryEntry.get_AdsObject()
 at System.DirectoryServices.PropertyValueCollection.PopulateList()
 at System.DirectoryServices.PropertyValueCollection..ctor(DirectoryEntry entry, String propertyName)
 at System.DirectoryServices.PropertyCollection.get_Item(String propertyName)
 at System.DirectoryServices.AccountManagement.PrincipalContext.DoLDAPDirectoryInitNoContainer()
 at System.DirectoryServices.AccountManagement.PrincipalContext.DoDomainInit()
 at System.DirectoryServices.AccountManagement.PrincipalContext.Initialize()
 at System.DirectoryServices.AccountManagement.PrincipalContext.get_QueryCtx()
 at System.DirectoryServices.AccountManagement.Principal.FindByIdentityWithTypeHelper(PrincipalContext context, Type principalType, Nullable`1 identityType, String identityValue, DateTime refDate)
 at System.DirectoryServices.AccountManagement.UserPrincipal.FindByIdentity(PrincipalContext context, String identityValue)