Well, which O/S are you running? pro makes this a little bit easier but you can do it on home as well. You simply have to turn on file and print sharing(which can be done from the security option by right clicking any folder). You need to specify the share name.
Once you do that, you go to the pc that you want to be able to access the folder you just shared. You right click on my computer and click map network drive. From here you select a drive letter(doesn't matter as long as it's not in use already), you type in the path which is equal to \\computerNameHere\FolderNameHere
Then it should ask you for the username and password you want to use. Without getting deep into security and folder permissions, just type in the Username and password of the main logon to that computer or the administrator account.