Well if you absolutely can not recover your music collection off your hard drive, you can recover it from your iPod using some 3rd party tools. The one I prefer is called:
SharePod
Others are:
YamiPod
vPod
(there are others that cost money)
I will add the following!!!
After you install iTunes on the new computer plug in your iPod, and make sure you choose to NOT have iTunes automatically update/control your iPod (set it to Manual).
You also need to make sure your iPod is in Hard Drive Mode to use these tools (default state for iPod Video, must be turned on for Nanos and older iPods)
This way iTunes will not wipe your iPod clean.
Once you use the tool to pull all your content off your iPod and onto your computer, then import it all into iTunes.
Once this is complete, and you are satisfied that everything is there, you can set yourself back to automatic from manual. And then re-sync your iPod to the new computer.
*****
Very important!!! If you use iTMS (iTunes Music Store) to purchase music/video then you need to be aware of the following. Your iTMS account is good on only up to 5 authorized computers. Your old computer is #1, your new computer will be #2. If you can not deauthorize your old computer, you will need to wait for Apple's yearly grace period. Then you can deauthorize all computers from your account and reauthorize the new one that was #2 to be #1.