To properly support Microsoft's SMB protocol, an IP Masq module would need to be written but there are three viable work-arounds. For more details, please see this Microsoft KnowledgeBase article.
The first way to work around this problem is to configure IPPORTFW from Section 6.7 and portfw TCP ports 137, 138, and 139 to the internal Windows machine's IP address. Though this solution works, it would only work for ONE internal machine.
The second solution is to install and configure Samba on the Linux MASQ server. With Samba running, you can then map your internal Windows File and Print shares onto the Samba server. Then, you can mount these newly mounted SMB shares to all of your external clients. Configuring Samba is fully covered in a HOWTO found in a Linux Documentation Project and in the TrinityOS document as well.
The third solution is to configure a VPN (virtual private network) between the two Windows machines or between the two networks. This can either be done via the PPTP or IPSEC VPN solutions. There is a Section 7.35 patch for Linux and also a full IPSEC implementation available for both 2.0.x and 2.2.x kernels. This solution would probably be the most reliable and secure method of all three solutions.
All of these solutions are NOT covered by this HOWTO. I recommend that you look at the TrinityOS documentation for IPSEC help and John Hardin's PPTP page for more information.
Also PLEASE understand that Microsoft's SMB protocol is VERY insecure. Because of this, running either Microsoft File and Print sharing or Windows Domain login traffic over the Internet without any encryption is a VERY BAD idea.