Is it possible to use mod_authnz_ldap to protect a virtual directory, but without prompting the user for his username?
I'd just like to use his windows login and check if it exists on the LDAP server (NOVELL).
No it is not possible. The normal SSO (single-sign-on) method is GSSAPI (usually with a Kerberos implentation). Both Internet Explorer and Firefox support the GSSAPI. Novell's "eDirectory" server seems to as well, though I've never set it up.