this may help quoted from http://www25.brinkster.com/ChicagoTech
Can't access some web sites
Symptoms: 1) If you're using MS ICS or Windows server as router to access the Internet, your clients may not be able to access certain web sites. However, the ICS host or the server doesn't have this problem.
2) After installed a new router or cable modem replacing dial-up to access the Internet, you can't access some web sites.
3) You also find that most web sites you can't access are block to ping or they are security sites using HTTPS.
Causes: 1) The problem occurs because many web servers block ICMP messages and the packets are dropped. As a result, the requested web site doesn't load.
2) This problem is caused also by an incompatible MTU networking setting. If you use MS ICS, you may want to configure all your Client computers to use the new, lower MTU as the default for all Internet communication. To modify the MTU size, locate the following registry key: HKEY_LOCAL_MACHINE\ SYSTEM\ CurrentControlSet\ Services\ Tcpip\ Parameters\ Interfaces. If you use a router like Cisco DSL Router, you may want to adjusting the PPPoE MTU Size. For example, to adjusting the PPPoE MTU Size on the Cisco DSL Router, do
ip address <ip address> <subnet mask>
ip adjust-mss 1452