Assuming the computers are using Windows 7 or Windows 8, a Windows Homegroup is one of the easiest ways to share files between them. Windows home networking has been extremely complicated to configure in the past, but Homegroup is easy to set up. Just create a Homegroup from the Homegroup option within Windows Explorer (File Explorer on Windows 8) and you’ll get a password. Enter that password on nearby computers and they can join your Homegroup. They’ll then have access to your shared files when they’re on the same network — you can select the libraries you want to share while creating a Homegroup.
Someone using the other PC will just have to select the Homegroup option in their file manager, browse your shared files, and download them to their computer. If a guest comes over, you don’t have to do any complicated Windows networking configuration on their PC — just give them the password to the Homegroup and they can quickly join it.