The nobody user is the linux equivalent to the "guest" user in windows.
Could you list the permissions of the directory being shared?
ls -ld /Files/Server/mp3/
The user nobody needs write permission on the directory itself to be able to delete files. Deleting a file edits the directory (a file to the kernel) and not to the file itself.
If you were to use the default "user" level security model, you would create your shared directory with rwxt permissions. The "t" (sticky) bit prevents one user from deleted another users files. I wonder if using "share" level security, and "guest" only causes the permissions to deny deleting. In this case, since you have only one user, you may not want to have the sticky bit set. You may want "rwx" permissions for user, group and other, but no stick bit. I don't have a share level security mode share on my desktop, so I can't test it from my laptop. ( Plus I only boot it up into Vista about once a month for updates)
For a globally writeable share, I would use the default "user" level security model, and the global option:
map to guest = bad user
I would create the share with the option:
allow guest = yes (same as public = yes)
This would retain ownership for authenticated users. For non-authenticated users (e.g. bad user from the global option) the username and group name would be "nobody". Setting the permissions of the directory being shared with "sudo chmod a=rwxt" would use the same permissions as the /tmp directory. The sticky bit prevents one user from deleting another users files. It is up to the user whether they want to change the "rw" permissions of the file depending on whether they want anyone else to be able to read or edit the file.