Client cannot create TCP connection to 80 port of Windows Server 2003.

[Morgan Cheng]

I have one Windows Server 2003 + IIS6 deploy-ed in a private network.
The IP address is 192.168.3.206. The gateway 192.168.3.254 do health
monitoring by try to connect 80 port of the server every 10 seconds.
It turns out that server doesn't respond TCP ACK to incoming TCP SYN
request.

The Windows firewall is turned off, should be no other anti-attack
software installed.
What kind of problem this might be?

Thanks,