Sorry about your computer troubles. I've had a Mac in some form since I was about 10 (my dad is a computer geek so we always had one at home). I had always been in the habit of doing routine maintenance with Norton Utilities or some such program, so last year I thought I'd do the same with my relatively new G5 using TechTools. Well, turns out that OS 10.4 (Tiger) has a lot of those utility functions built in, so when I clicked the button that said "repair" I was actually hitting a button that meant "erase"
Anyway, I feel your pain - and I agree that you should always keep the really important stuff backed up, even if you think it will never happen to you. Luckily, I had my whole iTunes library backed up on my iPod, and there are programs out there that will let you rip the songs off an iPod and back into your iTunes folder.
Also, there's a way to deauthorize computers in your iTunes account options. Go to the iTunes Music Store window and click on the button in the upper right hand corner to sign in. There is a button to deauthorize all computers on that account. Apparently you can only do this once per year.
I hope your new computer consoles you.