Does Active Directory (W2K3) and ADAM (W2K3) support or not StartTLS?
Yes. they do.
i have sucessfully connected to Active Directory and ADAM via LDAPAdministrator and LDAPS, but i can't connect via StartTLS.
SSL and TLS are tough issues, they require certain knowledge to be able to perform necessary actions on the server side in order to install SSL support. If you aren't familiar with X.509 and SSL in particular, you expose yourself to a high risk of making a mistake while issuing a server certificate or adjusting your server configuration, which will make it impossible to connect to the server. Our experience shows that in 99% of cases SSL problems appear due to the server-side issues. To troubleshoot your SSL issues, try running your server or SCHANNEL API (an SSL implementation used by LDAP Administrator) in the debug mode
http://support.microsoft.com/?id=260729.