One of the niftier features of Windows 7 is the ease of file sharing between a home network, thanks to the Homegroups feature of the OS. Once set up, a Homegroup easily allows PCs to share files between PCs without going through the hassles of messing around with the file sharing permissions. It's mostly an automated process, with Windows easily detecting if there's an existing Homegroup in place and prompting you to join one, or if there's none and asking if you would like to set one up.
So there's really no need to mess around with file sharing permissions, workgroup settings or mapping drives just so you can share your files on your home network. And the best part? Homegroup is as mentioned, really simple to set up. Here's a quick pictorial guide below on getting started.
Note that if there's already a Homegroup set up on the network you won't be able create one. Instead, you can only share the group. So yes, you can have only one Homegroup - reason being Microsoft wants to simplify sharing for home users. Another caveat for Homegroup to function is that all other systems should be using Windows 7 as well. So it's probably not something you can use immediately until you've upgraded your other systems.