what does your fstab look like? i have a similar setup and even when i have samba setup correctly you must make sure the vfat drive has permissions set correctly.
here are mine as examples..
/dev/hda6 swap swap defaults 0 0
/dev/hda5 / reiserfs defaults 1 1
/dev/hdc1 /mnt/hdc1 vfat umask=0000 0 0
/dev/hda1 /mnt/hda1 ntfs ro 1 0
/dev/hdi1 /mnt/hdi1 vfat umask=0000 0 0
/dev/md0 /mnt/md0 reiserfs rw 1 0
/dev/cdrom /mnt/cdrom iso9660 noauto,owner,ro 0 0
devpts /dev/pts devpts gid=5,mode=620 0 0
proc /proc proc defaults 0 0
notice the umask=0000 on my vfat drives. sure its not the most secure way to do things, but i had to get this working quick one day and i just kinda left it. but those permissions allow read/write to the vfat drives. now for smb.conf...
workgroup = WORKGROUP
encrypt passwords = yes
security = share
both vfat shares are defined as writable and guest ok. that should let your client computers login just fine. feel free to change the writable permission but in case you want to move some music over to the drive at a later date its nice to have on. if they dont have the guest account enabled on the windows machines, enable it and it should log you on fine.
all this being said the above configuration should work without logging in, but it is by no means the safest way. i just recently set this up myself on a new server box and just did it fast so it would work, so give those changes a shot and see if it works, then go back and close things down a bit so your files arent so open to everyone-The_Nerd