IPSEC prevents Windows Server 2003 to connect network

Problem: Network connection fails completely on Windows Small Business Server SBS 2003. Can't ping. Can't browse the Internet.
Reason: IPSEC service doesn't start. (Usually after SP2 install)

Problem: IPSEC service doesn't start because of some "only one" error on "port"
Solution:
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Reserved Ports
add:
4500-4500
to the list
restart DNS.
start IPSEC.
You should have network connection.