autorisation of computer

I want to synchronize my Ipad and my computer.

Computer says:

The Ipad cannot be synchronized because this computer is not authorised to the apps you have bougt on this Ipad.

Please autorise your computer.

I do as I'm told.

Computer says: your computer is already autorised

I try to sychronze

The Ipad cannot be... (loop)

What might have happened: This is the same computer but with another disk.

Or: I hav changed my Apple ID and password.

In any case, what ca I do.