according to phpinfo()
session.save_path = C:\php\session
session.cookie_path = C:\php\session
NTFS permissions on C:\php\session are as follows.
HC7Group (group name of all IIS users) read + ex, list, read, write.
Administrators = full control
IIS guest = read + ex, list, read, write.
Network service= read + ex, list, read, write.
system = full control.
default iis user is IUSR_TEMPLATE