Sharing Tab? Oh, do you mean the one that turns on the option to share files? Go into the control panel, open the networking thingy, and there should be a button for "File and Printer Sharing." Click that and check the box for letting other computers access your drives. Click OK, then check your bindings for TCP/IP on each of your network adapters. If you use a different adapter for the internet than you use for the LAN (like dial-up), unbind the file and printer sharing for that adapter (you don't want your shared files exposed to the world, do you?).
You'll probably have to restart once that's all done (yay Windows) and then you can go into your file manager, right-click on any drive, folder, or file, and select "Sharing..."