If you still have access to your Old computer...
From your OLD computer...
Copy your ENTIRE iTunes FOLDER to an External Drive... and then from the External Drive to your New Computer..
Full Details Here > http://support.apple.com/kb/HT1751
If you don't have access to the old computer see Recover your iTunes library from your iPod or iOS device. If you do there may still be useful tips in there to prevent loss of contact & calendar information when switching to a new machine.