I have no problems accessing AD with code like:
DirectoryEntry ldapConnection = new DirectoryEntry (ldap, ldapUser,
ldapPassword);
Is there a way to access AD via DirectoryEntry without having to provide a user name / password. In this particular case, I just need to read some attributes.
By default, Server 2003 and later do not allow anonymous queries against AD (for the most part). Make sure your AD Server is properly configured to allow anonymous access.
How-to fix (for Win 2k8):
http://technet.microsoft.com/en-us/library/cc816788(v=ws.10).aspx