Why is samba denying me access to my share?

On my CentOS 5.2 box running Samba (3.0.33-3.29) I created a folder called /upload.

In samba I configured a share like this:

[upload]
        comment = upload folder
        path = /upload
        valid users = kevin root
        public = yes
        writable = yes
        browsable = yes
        create mask = 0777
        directory mask = 0777
        guest ok = yes

I chown'd the /upload folder to my account 'kevin' and checked that I could create files and folders via the shell.

I can browse to the machine from Windows 7, authenticate as 'kevin' and see my home directory share and the upload share but I can't access them.

Windows reports:

Network Error

Windows cannot access \\cos-01\upload

Check the spelling of the name. Otherwise, there might be a problem with your network. To try to identify and resolve network problems, click Diagnose.

Error code: 0x80070035
The network path was not found.

This is a check list of what I've done:

• the account kevin was added to samba using smbpasswd -a kevin and setting my password at the same time. The samba and centos passwords are both the same.
• the server name cos-01 is in the /etc/hosts file i.e.

172.0.0.1  localhost localhost.localdomain
172.17.3.90  cos-01

• I've also set the netbios name in /etc/samba/smb.conf
• I configured Windows 7's LAN Manager authentication level to "Send LM & NTLM - use NTLMv2 session security if negotiated"

Update:

I tried accessing the share by both IP address and server name i.e. \cos-01\upload or \172.16.3.90\upload. In both cases I get the same error as detailed above.

I checked the /var/log/samba/smbd.log logfile and see lots of:

[2010/07/02 16:56:10, 0] smbd/service.c:make_connection_snum(1013)
  '/upload' does not exist or permission denied when connecting to [upload] Error was Permission denied

Is SELinux active? If it is, then you can make it accessible by setting the type to public_content_t. If samba should be able to write to it, then set the type to public_content_rw_t. Note that if you do the latter, you will also need to tell SELinux about this; my system-config-selinux has a boolean for this: Allow Samba to write files in directories labeled public_content_rw_t