Are you not able to get to the old computer at all? I did find instructions on moving your music using your iPod in HD mode, but you have to be able to get to your library on your existing machine.
Per the instructions...
"While you can use the Transfer Purchases from iPod option under the File menu to copy iTunes Store purchases from the iPod to another computer, you can't transfer songs from other sources from the iPod library to iTunes, so you'll need to follow the steps below to use iPod as a hard disk to transfer music files from one computer to another."
If you are able to get to the PC, let me know and I can pass along the instructions.
--Daneen