The reason is Vista follows the base permissions model from Windows 2003, which doesn't grant broad permissions to the Everyone group for security reasons.
Keep in mind there's a difference between NTFS (file level permissions) and share permissions (network access permissions). You must have rights through BOTH NTFS & Share permissions to access something over the network.
Typically, this is best accomplished by granting Everyone share permissions, and restricting permmisions using NTFS, since NTFS will trump share permissions anyway.
This permissions model allows flexibility to allow certain people to access data locally, but not over a network should you ever want that, but can be a bit confusing for people not familiar with how it works.
"Enough, enough bowing down to disillusion!
Hats off & applause to rogues & evolution!
The ripple effect is too good not to mention.
If you’re not affected, you’re not paying attention!"