I am assuming you have a router? And both machines are plugged into the router?
First make sure both machines are in the same workgroup, and that file sharing is enabled.
Then on the pc you want to copy *from* open a run box (Windows Key + R) and type in \\machinenameoftheothercomputer\C$\
After authenticating you will have access to the C Drive, and will be able to navigate to where you want the files copied. Then just copy / paste.