Hi, I also had something like that. I reinstalled 7.5 on 7.5 and I received an error-message with the next try to log in. So I tried to reinstall again, but pressed the "repair" button when I was asked what to do. After that iTunes store told me that my data has changed and I need to check it. I did that and now I am able to buy music again. The only thing that is still bothering me is, that I have to "authorize" my computer every time new when I close iTunes before I can listen to the music that I bought at the iTunes store.
I tried this... it didnt work... I'm still getting the "This Apple ID has not yet been used with the iTunes Store" message, which is rubbish... because I have 20 songs in my purchased music section, so I have used it! I've just tried 're-registering' the account, i.e. putting my back details in it again and im now logged in. The most annoying thing though... I registered a 25 pound gift voucher last time I logged on, and it's conveniently 'forgotten' this. I am SOOOOOOO cross!!!!!!!