Hello Paolo,
the PCNS service has a queue on the Domain Controller. The %windir%\System32\PCNS folder is the folder that is the PCNS queue for the Password Change Notfication Service (PCNS).
For more details about PCNS i recommend MIIS_2003_Password_Synchronization_Step_by_Step.doc in the Microsoft® Identity Integration Server 2003 Scenarios collection (
http://www.microsoft.com/downloads/details.aspx?FamilyId=15032653-D78E-4D9D-9E48-6CF0AE0C369C&displaylang=en)
/Matthias