Ubuntu Samba server not discovered by Windows 7

Rather than reduce the security offered by Windows, you can add this to smb.conf:

local master = yes
preferred master = yes

This will make your Samba server appear in the Windows 7 Network browser.

For added fun, you can also add

ntlm auth = no
lanman auth = no #the default is no.

but this will require you to add the

sec=ntlmv2

This article might help: Get Vista and Samba to work.
(Although written for Vista, it also applies to Windows 7.)

If the cause for the problem is NTLMv2 authentication, this article describes how to make Windows 7 negotiate with Samba for a common authentication protocol.