App authorisation for my newly re-installed computer

Hi! I have some questions about computer authorisation for my ithouch.
I just re-installed my operating system and installed itune. When I plugged my itouch in, it said that I've allowed 4 from 5 computer to my iTouch. The fact is, as I said above, I've installed window + itunes. What can I do to fix the number of used authority? I am not sure that if I install my computer next time, it will be the 5th time and i will definitely run out off authorisation.

Thanks in advance for your help.
Kewalee K.