Failing that, is there a way to deauthorise all PC's but the one you're accessing the iTunes account on?
Only when you've reached your limit of 5 authorisations. If you go to your account you get the option to de-authorise all machines. Then you re-authorise the machine you're accessing your account with.
Thanks
I'll do that.
I thought you just had to : Open iTunes > Advanced > De-authorize
![]()
That deauthorizes just the computer you're on. But if you no longer have access to the other authorized computers (sold them, forget to deauthorize before reinstalling an OS, etc.) you can deauthorize all of them at once.