Active Directory/LDAP :: Calling Active Directory From Web Service?

i have an webservcie that is on my localhost(windows xp).this webservice is authenticating the user from active directory when i am testing from my system it is working fine but when i tried from other system of same domain,i am getting the users identity in webservice ,but it is giving the error while directory searching.The Authentication mechanism is unknown.The code in my webservice is

DirectoryEntry de =
new
DirectoryEntry(domain, admusername, admpassword,
AuthenticationTypes.Secure);

[code]...

1) Anonymous user disabled:

2)Integrated WindowsAuthenticated checked

3)digest authentication checked Web Config:

1)Impersonation=true;

2)authentication mode=windows;