For Windows 7 go to Control Panel > Network and Sharing Center > Advanced Sharing Settings
Turn on Network Discovery, File and Printer Sharing the rest of the options depend on how you setup your network.
Once that is on you have to share the folder. Find the folder you want to share
Right click the folder and go to Properties > Sharing > Advanced Sharing
Check the box that says Share this folder
Give it a Share name. You can also control who has access to this share by clicking on the Permissions button.
You can also set permissions on who can access the files in the shared folder by going to the Security Tab and then Edit
Once you done hit Close.
Now the file is shared. To access the share from another computer on the network, Open up Computer and in the address bar Type \\ComputerNameWithTheShare\ShareName
So if the computer name with the share on it was called AndrewPC and I shared a folder called Movies i could access it by typing into the address bar
You can also create a shortcut to a share so you can put it on your desktop. To do this Right click on the desktop > New > Short Cut
Where is says type the location of this item put the same
As above. This way you don't need to type it each time.
If you are getting errors, check the share and file permissions as well as the settings on the control panel we set at the start.
(Will have to make a hub about it later ;))