TCP packets don't flow from external hosts to WinVista clients behind

[MIZ0]

 > Could be TCP window scaling. See
 > http://en.wikipedia.org/wiki/TCP_window_scale_option
 > Or the plain old PMTUD problem described in
 > 
http://www.cisco.com/en/US/tech/tk870/tk877/tk880/technologies_tech_note09186a008011a218.shtml#backinfo
 >
 > =Adriaan=

Nothing helps.
I've tried to change client's mtu, even shrinked packets with ng_tcpmss 
- no effect.
I don't understand why freebsd machines from internal network can't 
establish  any TCP connection to external net too.
Can ipfw or netgraph detect client's OS type and allow only Windows XP ? =))