You can definitely get back all your purchases. I've never done it, but that's because I don't really buy anything from the iTunes store.

Tony, let us now what happens when you look at the problem from a user account with clean iTunes prefs. From what I recall, you should be able to move to a new computer without formatting an iPod/iPhone, so you should also be able to go to a new user account - just set it up to connect to the iTunes store with the same credentials you've always used.