Nagios Support Forum

I'm not sure how it got there, since you are using xinetd, but I can't imagine it will cause too many issues provided you only have either xinetd or nrpe services running, personally go with xinetd.

ok, do i have to define somewhere to use xinetd?

You likely won't need to change anything at this point, xinetd is smart enough to look at it's defined configurations and initialize what is contained within. If you issue:
It should re-initialize under xinetd.