I like using rsync to copy files between PCs, even on Windows. You get all the niceness of copying when the files are in use, and then only doing a small synchronization when they're not, to get a complete copy.Doesn't handle Windows file permissions well though.
Enter Robocopy (available in the Resource Kit). But I've already got the files?
robocopy source dest /s /copy:sou /is
Copies only security, owner and auditing information. Very handy.