Win user: <root>\Stamps
Win program: <root>\plug_ins\Annotations\Stamps\ENU
Roots:
Windows user root: \Documents and Settings\<user>\Application Data\Adobe\Acrobat\<version>
Windows program root: \Program Files\Adobe\<Acrobat version>\Acrobat\
Win 7 roaming: \Users\<user.domain>\AppData\Roaming\Adobe\Acrobat\<version>\
hth,
Ben