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).

If you've got an iCloud account, you can remove it from all authorized systems by using this way:

Go to: www.icloud.com
Sign to your account from the web page.
Go to iCloud Settings (by clicking on your account name at the top-right corner and clicking on "iCloud Settings".)
Go to "Data & Security" settings, which is a subset of "Advanced" settings.
Go to the tab "Sign Out Everywhere"
As you see in this tab we have a link says "Sign out of all browsers" and that's exactly what we need, so click on this link.
Then iCloud will just warn you and ask "are you sure you want to do this?", so just sign out.

Remember that if you click "Sign Out" your account will de-authorize from all systems you are authorize with; it means you have to sign in your Apple ID the next time you want to work with iTunes.

Then you just have to authorize your ID with your new system. If you want to authorize follow these steps:
Open iTunes
Open the tab named account (by clicking on the small circle on the top-right)
Chose the "Account Info"
Enter your Apple ID and password then chose "View Account"