Windows has this option built in already.
Following the instructions on that link will set up almost any directory on your PC and will allow it to be shared with any PC on your LAN, regardless of OS. In fact, once you set it up, you can access that folder from PC, Mac, iOS, Android, XBox360/One, PS3/4, and more.
If you are looking for something different than this, can you provide more details?
Windows has a file synchronization system built in as well, that, in my experience seems to work a bit more like Dropbox than simply sharing a folder. See this page for details.
Also, here is a LifeHacker Article that may help as well.