iPod Touch is randomly unregistered, can't sync without deleting content?

I was having issues updating my iPod; my old computer died, and so I put iTunes on my new one. Now, however, although my computer is authorized, it will not sync any app or video purchases. As such, I can't update my software without deleting every single app I have ever purchased. Additionally, when I went to fastrack, it randomly started telling me that my iPod was never registered with my Apple ID (which is literally impossible, because otherwise I wouldn't be able to purchase content). Can someone please help me? I've got somewhere around 70$ in apps, and even more in music and videos that are apparently irreplaceable.