Have you created a Samba account on your server machine? (sudo smbpasswd -a
username)

Then use those login credentials from your Windows 7 PC.

I have a very similar setup: USB NTFS drive plugged in to a Ubuntu machine
running Samba, with other desktops running Ubuntu or Windows (XP and Vista) on a
LAN. I have no problems accessing shared folders on the USB drive from the
Windows boxes.

I did have to disable the Windows firewall on my old XP box to get to my shared
folders. But that was not specific to the USB drive.