Here is some info that should get you started, I am assuming Win XP and Mac OS X. This process works regardless of whether you are connecting by ethernet or wireless router, or even through a crossover.
For the Mac to access files on the PC:
Make sure your PC has sharing enabled (right click on the folder and click 'sharing...' to get you started).
Get the ip address of the PC by going Start -> Run and typing "cmd" and typing the command "ipconfig"
On the Mac, go to the Finder and choose the menu Go -> Connect to Server
In this window, type: "smb://" and the ip address of the PC. For example, if your PC has the ip address "192.168.1.32" then the address will be "smb://192.168.1.32". If you are going to be using this regularly, you can click the 'plus' icon to add it to the favorites area.
A window will pop up asking you what workgroup to connect to, a username and password. Type in "MSHOME" for the workgroup (it may come up by default), and your PC username and password in the other two fields.
After this, another window will pop up asking you to select a share mount. The default is "SharedDocs" but you can add others to this list by creating separate shared folders on the Windows machine.
After clicking OK, the folder will be mounted on the Mac desktop just as if it were an external drive. Just make sure to unmount the folder (by dragging it to the trash which will change to "eject") before restarting the PC or disconnecting either machine from the network.