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

[mich.admin at mail.ru]

Hello...
I've got a strange trouble...
FreeBSD 6.2 amd64 as nat-router:
rl0 85.249.249.249 -> ISP
fxp0 10.0.0.1 -> My Internal Net
natd/ng_nat
ipfw: allow all from any to any

WinXP client machines work fine behind nat, but WinVista, FreeBSD (5.5/6.2) clients don't.
It's very strange but it's unable to establish any TCP internet connection from non WinXP host. ICMP and UDP packets flows normally.
For example, I can ping host ya.ru, get DNS-reply from internet servers, play games via UDP, but i can't view web pages, open shh session and just can't "telnet" to any port!!!!
Traffic from/to my net is not filtering by firewalls at all. I've tested it with natd ang ng_nat - there's no difference! I've tried to solve this problem during last week =( Unfortunately, i can't. Now i'm going to try using pf or ipfilter instead of ipfw or ever reinstall server OS and may be change freebsd architecture to i386.

Any ideas ?
P.S.Sorry for my bad english.