How Do I Get A User's Domain?

I need to provide a UI to get the information to add a windows login to a SqlServer database. The CREATE LOGIN Sql statment requires the user name as "DomainNameUserName". I can get a list of users in XML using the following code:

public static XmlDocument GetAllADDomainUsers(string DomainPath)
{
string domain;
XmlDocument doc = new XmlDocument();
doc.LoadXml("<users/>");
XmlElement elem;

DirectoryEntry searchRoot;

ArrayList allUsers = new ArrayList();

if (DomainPath.Length == 0)
{
DirectoryEntry entryRoot = new DirectoryEntry("LDAP://RootDSE");
domain = entryRoot.Properties["defaultNamingContext"][0].ToString();
}
else
domain = DomainPath;

searchRoot = new DirectoryEntry("LDAP://" + domain);

DirectorySearcher search = new DirectorySearcher(searchRoot);
search.Filter = "(&(objectClass=user)(objectCategory=person))";
search.PropertiesToLoad.Add("samaccountname");
search.PropertiesToLoad.Add("distinguishedname");
search.Sort.PropertyName = "samaccountname";
search.Sort.Direction = SortDirection.Ascending;

SearchResult result;
SearchResultCollection resultCol = search.FindAll();
if (resultCol != null)
{
for(int counter=0; counter < resultCol.Count; counter++)
{
result = resultCol[counter];
if (result.Properties.Contains("samaccountname"))
{
elem = doc.CreateElement("user");
doc.DocumentElement.AppendChild(elem);
elem.SetAttribute("name", (String)result.Properties["samaccountname"][0]);
elem.SetAttribute("distinguishedName", (String)result.Properties["distinguishedname"][0]);
}
}
}
return doc;
}


This works for listing the names but how do I get the NetBIOS domain name for a selected user as required by SqlServer? I have tried using TranslateName from secur32.dll. That works on some machines but for some reason on other machines, it returns a blank. Is there another way?

Thanks for your help,
Rob