Andrea,
He is looking for an SSO solution that uses AD as an authentication server (via. SNC interfaces). There are many of these available, either from SAP or from SAP partners. Looks like he wants one for free since he said he doesn't want to buy third party s/w.
Renato,
If you don't want to spend money on a product, you can build your own using open source Kerberos libraries. Other SAP customers have done this. I personally wouldn't recommend it, but you can if you wish.
Thanks,
Tim