Problem on ICMP Echo : all ip checked reply when firewall is disable.

Hello,
In a Homemade soft, i check avaible ip on the network by checking the reply
of a ping.
I use ICMP Echo command of IP Helper api.
It works perfectly on vista when firewall is enable but when i disable it
something strange happen.
My soft scan an IP range, and in this configuration, firewall disable, it
find reply on all IP checked.
To verify this result, i try manualy to check those IP by ping command in
cmd and the result is none IP reply.
It's very strange, may there is a problem in default ICMP configuration when
firewall is disable.
After a long search on net, i don't realy find something that can explain
the origin of this problem.
So i come here to ask you some help.
May someone can have an idea to resolve it.
I'm open on all opinion or advice.
Thanks for all.
Best regards