You will need to post the Global section of smb.conf and the share definition. If you have "map bad user = guest" and "guest ok = yes" in the share definition, then it doesn't matter what credentials you enter.
The Linux equivalent of the window's user "guest" is the Linux user "nobody". A globally writable share should be created owned by nobody, with rwx permissions, and the sticky bit should be set.
sudo mkdir /srv/samba/sharename
sudo chown nobody.nobody /srv/samba/sharename
sudo chmod a=rwxt /srv/samba/sharename
If you use the default "Security = user", and you log in as user who has credentials in samba & linux, then the ownership of files shared will be of that user. If you aren't a Linux/Samba user, then the file created will be owned by "nobody". The sticky bit on the directory prevents one user from deleting a file owned by another user.
If you have a user on Linux, you use the `smbpasswd' program to create a samba user with the same username and passwd.
This assumes that your server uses the 'smbpasswd' file for storing credentials. There are other methods, such as using a different backend, and using Samba as a domain controller. But it seems you are using pretty much the defaults, and I assumed you aren't running a domain controller (which XP home can't join anyway).