Make sure you're two computers are on the same network, in the same IP address range (typically 192.168.0.x and 192.168.0.y).
Also, if you use an iPhone, make sure you back up all your contacts to a mail program or something like Outlook or Entourage.
I used Remote Desktop for Mac to access the PC where I transferred my iTunes files from the PC to the new Mac. You don't need this to move your iTunes, just need to have the PC and the Mac running at the same time on the same network. It was just handy to use to do this tutorial. If you want to get Remote Desktop Client for Mac, go to http://bit.ly/gBxaY and scroll to the boxes at the bottom of the page.
