For security reasons, you should never try to access a Samba share using the root account. You should always use a less-privileged account that is registered with Samba.
The entire text of the very useful O'Reilly reference _Using_Samba_ is available for free online:
http://www.oreilly.com/catalog/samba/chapter/book/
There are options in the file smb.conf (normally located in /etc/samba or /etc/smb depending on which distro you're using) that you can set to allow/disallow access for different user types, including guest accounts, administrative accounts, and even simply anonymous access without signing in -- though, if possible, it's better to simply have Samba access accounts set up that match the usernames (on client machines) of all users you want to allow to have access.
I typically do as swdswan suggests, matching Samba users with system users. It minimizes administrative overhead. For some uses of Samba this isn't a practical solution, but often it is -- particularly if you want every Samba user to have a directory of his or her own on the Samba machine.
I recommend you sign up on a LUG (Linux User Group), if you're going to be working with Linux much. Infoworld rated the Linux user community the best tech support organization in the industry a few years ago, and with good reason. In fact, based on my own extensive experience, I'd say the Linux user community is still the best tech support organization in the world, and the biggest reason for this is LUGs. Subscribe to a LUG mailing list, and you can get a wealth of knowledge for free through email, usually much more quickly than you'd ever get any help at all from other tech support channels.
You can find a LUG near you here:
http://www.linux.org/groups/