Can I deauthorize all computers on iTunes, without being on an authorized computer?
I solved it myself.

For reference here's the steps I followed.

On the iTunes menu open Store->View Account, then in Account Summary click "Deauthorize all computer/devices/whatever.." (I'm translating in english the steps)

I wasn't able to do this on the menu with Store->Remove Authorization, because it was asking me to be authorized first to do it (and I already had 5 authorized computers).