mount error 13 = Permission denied -> what is the correct samba configuration for guest access from windows client

hi!

I have configured samba with different shares, of which one is accessible for guest from client windows.
the output of testparm:
Load smb config files from /etc/samba/smb.conf
Processing section "[install]"
Processing section "[printers]"
Processing section "[print$]"
Loaded services file OK.
Server role: ROLE_STANDALONE

[global]
	workgroup = NETZWERK
	server string = %h server (Samba, Ubuntu 8.04 LTS)
	obey pam restrictions = Yes
	passdb backend = tdbsam
	pam password change = Yes
	passwd program = /usr/bin/passwd %u
	passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
	unix password sync = Yes
	syslog = 0
	log file = /var/log/samba/log.%m
	max log size = 1000
	unix extensions = No
	dns proxy = No
	usershare allow guests = Yes
	panic action = /usr/share/samba/panic-action %d
	invalid users = root

the share is defined like this:
[install]
	comment = Unattended 4.8
	path = /mnt/iso/unattended-4.8/install
	guest ok = Yes
	locking = No

when I execute the command on the client machine 
user@client:~$ sudo mount -t cifs -o username=guest //<ip-samba-server>/install /mnt/share/
the following error is returned
mount error 13 = Permission denied
Refer to the mount.cifs(8) manual page (e.g.man mount.cifs) 

in the global section of smb.conf I've also defined
guest account   = nouser
unix extensions = no


I'm wondering why this is not displayed in the output of testparm?

the samba server is running on ubuntu 8.04 LTS.
there's no guest user account defined.
instead the user "nouser" and the group "nogroup" exist.

therefore I cannot passwd -a guest, this will fail.

question:
what do I have to configure in smb.conf in order to allow guest access from a windows client?
how do I define a password for this user "guest"? or is no password required?

THX

if I execute command
user@client:~$ sudo mount -t cifs -o username=<sambauser> //<ip-samba-server>/install /mnt/share/
everthing works after entering the password.