Well, you're really asking how to share a folder in Vista, right? I don't have Vista, but in XP you'd right click on the folder you want to share, click on Sharing and Security, and then choose Share This Folder. You might have to Ok a security warning, and you might not have this available depending on the version of Vista you're running.
Try googling how to share a folder in Vista. Then your Mac should just see the PC in the Finder sidebar. It will work best if both Mac and PC have the same workgroup assigned and if both have the same user ID and passwords.