Before version 10.2.0, Mac OS X did have a windows network client but it was not really presented to the user as a feature, or even well documented. It used the Samba client to connect to a windows server, but because no samba browser had been included you had to actually know the address of the system you wanted to connect to.
You can connect onto a Windows server in 10.1.x by doing this:
- In finder, choose Go -> Connect to server.
- Enter the address like this:
smb://1.2.3.4/sharename
where 1.2.3.4 is you put in the IP address of the server.
You can also connect using workgroup and computer name, in a similar way.
Of course, all of this was automated with the introduction of 10.2. In the meantime, what you want to install is the Samba sharing package (which allows 10.1 to act as a Windows share) and some kind of Samba/Windows network browser ... there were a few of these around before 10.2 was released. You can probably find what you want on Version-Tracker or on Apple's Mac OS X downloads page.