Ok its fairly simple to install and get running, the hardest bit (which isnt really that hard) is configuring Samba itself and setting up your shares. Heres a rundown of what you need to do.
1. Install MacPorts. Google MacPorts, you'll find the site and instructions for installing it. Note that it does require the latest XCode to be installed first.
2. Open a terminal and type 'sudo port install samba3'
3. Navigate to /opt/local/etc/samba3 and modify the smb.conf file to your needs. This is where you setup your shares. Theres plenty of documentation on the web for how to set this up correctly if you get stuck.
4. Make sure that File Sharing is disabled in System Preferences (to prevent Lion's SMB from conflicting with the one you just installed).
5. Once everything is setup you'll need to launch the smbd and nmbd services as deamons, do this by typing the following in Terminal:
sudo /opt/local/sbin/smbd -D && /opt/local/sbin/nmbd -D
If all went well, you should see smbd running when you look in Activity Monitor. If you setup your shares correctly in the smb.conf file above, you should be able to connect to them from your Windows machine as normal. Getting the shares all setup right is the hardest bit out of the whole thing.
6. The last thing to do is set it up so that the smb deamon is launched automatically when you boot, thus preventing you having to run it manually all the time. Best way to do this is using an app called Lingon which will help you setup a plist which will tell OSX to launch the smbd and nmd automatically for you. On my machine I have my own script launching on boot which takes care of launching smbd plus doing a few other little things, but you wont need to do that on your setup.
Theres a number of blog posts on the net that describe doing this in greater detail and which will probably be of more help than Ive been. I dont have the url's to hand but a quick google search should bring up some results. Im happy to help further if I can however.