I installed Windows 7 pro (which is really fast compared to Vista, it has to be said) and reinstalled itunes. Like a tool I forgot to move my music folder to my new hard drive (which was bought for this reason!) and lost it when formatting, after I discovered you can't upgrade from Vista home premium to Win7 pro, and have to do a full install (which was astonishingly quick compared to all other windows installations I've done. According to my mate who has an SSD drive, it took him less than 15 mins to install!).
Anyway, on installing itunes problem free, when I connected my iphone, it deleted EVERYTHING on the phone. Music, Apps, the lot. There was no option to keep them, import them or anything.
Looking back on it I THINK I needed to enter my account details then authorise the "new" PC (even though hardware is identical) to use it, THEN connect the iphone.
It didn't help matters that my cat had just been killed, and my latest photos of her were on it either, but somehow it didn't delete them, even though it said it had. I even had a wee cry at the time when I thought I'd lost them! :(
The licences are tied to your e-mail address so unless you authorise it first it wipes them from any device (great design there Steve)
Only thing you can is phone Apple and see if they can set the tracks to download again (quite why it won't do this anyway if the music is still on Apples' servers I don't know)
It's not the end of the world thankfully as I have the main part of what I currently like on my mp3 player (which I only remembered about an hour ago!). The rest of my music is on the shelf behind me. I'm off all week so I'll re-rip them.
Why didn't you have a backup? :P
Sorry to hear about your cat, I lost one of mine last year and the remaining one is getting old (19) and creaky. It's a bit of a surprise every morning when I get up and find she's still going.