I use iTunes (latest available build) on Windows 7 x64.
Whenever I buy apps for my iPhone, or download free updates to apps I already purchased, iTunes asks me to enter my Apple ID password, and my credit card confirmation number; everytime this happens, I also get an email from Apple saying "you purchased something from a device that has never been associated with your account, watch out because someone could be messing around with it".
I always used iTunes only on this computer, and I've tried logging out/in, and also unauthorizing the system and then authorizing it back again; but this keeps happening.
How can I fix this?