I used the search feature of this website, and found this link that explains your problem/fix completely.
I'll save you some work by quoting the relevant part here.
"This is what is in the "out of the box" minimal.cfg-sample file that comes with the nagios-2.0b2 and is most likely your problem.
Command to check to see if a host is "alive" (up) by pinging it
command_line /bin/check_ping -H $HOSTADDRESS$ -w 99,99% -c 100,100% -p 1
So if you used this sample, then you are going to have trouble, which is what I stated in your other thread. I have the new beta nagios working, and there are a few of these kind's of buggy items."