Here is a bit of useful code to fetch the list of all valid widows Users under current domain
string defaultDomain =
System.Environment.GetEnvironmentVariable("USERDOMAIN");
string queryString = string.Format( "select * from Win32_UserAccount where Domain = '{0}' ",defaultDomain);
SelectQuery query = new SelectQuery(queryString);
ManagementObjectSearcher searcher =
ManagementObjectSearcher(query);
ArrayList ExistingWindowsUsers = new ArrayList();
foreach (ManagementObject envVar in searcher.Get())
ExistingWindowsUsers.Add(envVar["Name"]);