Thanks my friends. Now I have understood that these scanners gives sometimes "false positives". Yes those are tests that have been done but the nikto's conclusion from site's response is not always correct. About double checking, it's a good idea but in the case I'm working on, it takes a very long time since the findings are several pages. That site is definitely vulnerable (I have some evidences) and I'm trying to scan it with other scanners too.
(By the way, being a little playful I will be so happy if some of you has time and likes to help me in fighting that site )
Thanks