I have posted this elsewhere, but it may fix your problem.
The "fix" that has sorted the Windows 7 Home Premium system's ability to map the Zentyal server drive is called a clean re-installation of the Win 7 system BUT with the initial installation selection made for a WORK network when first asked. Absolutely no other changes have been made to the Win 7 or Zentyal systems and it all worked "out of the box".
(It does not seem to matter what sort of Win 7 network you try after you have made the initial install as "home" network - it would seem that it will never again link to SAMBA if initially installed as other than WORK network).
No doubt there is some logic in this from the way Win 7 default permissions are lodged in the registry, but I lack both time and ability to find it. Perhaps it is linked to the "new" Win 7 feature of the "homegroup" - I read somewhere that MS have been made to implement SAMBA interoperability, so perhaps their "new" Win 7 "homegroup" is exempt from the earlier ruling.
As a bonus, this has also solved another annoying problem that I was having, in that I can now connect to an old printer that is networked off the USB on an equally old ASUS WL-500g router that is running SAMBA.
What is still "missing" on the Win 7 network list (although reachable via IP address) are a couple of Belkin F5D7633-4 routers although both the old ASUS router and the Billion MODEM 5200S-RC show up.
Perhaps if someone can confirm this "fix" (or even better if Zentyal and/or Ubuntu can reverse engineer it to see what is happening in the Win 7 registry) it could go in the FAQs.
That might even save someone else having to spend quite so much time googling the problem for a working solution and improve the take-up of Zentyal in a "mixed" small business network?