All Ports filtered, no OS identify

[dednj cjcb ekdj <netwiator () yahoo com>]

Hello everybody.
 
When I try nmap -sS -P0 192.168.1.100 , I get the response all ports are filtered.
 
The same result with -sW , or with -O  
 
I ´ve installed on 192.168.1.100 a PFW. 
 
But how can I get any information (OS, Service Pack) out, if even do not get any answer from the machine? Is there any 
trick or do I have forgotten something?
 
Thx in advance

                
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