Question : Some clients can not access certain websites.

At one office location, I have several users who can not access the following web sites:
www.weather.com
www.iowadnr.gov
www.msn.com
www.cargill.com
When they leave the office with their laptops, they can once again access these websites, but when they return to the office they can not.  Clients are getting IP addresses via DHCP.  Some clients at this office are able to access these sites with no problems.  All clients are hide NAT'ed behind the same external IP.  Sophos Antivirus (no firewall) and Windows Firewall is installed and active on all laptops.

What hasn't worked:
Connecting the laptop directly to the T1 router (managed by XO) with an external IP address.
Using Internet Explorer or Firefox.
Replacing the Check Point Edge Router with a new Check Point
Replacing the Check Point Edge Router with a new Buffalo Router
Setting the IP address of a failing laptop to the IP address of a working laptop.
Disabling Windows firewall.
Uninstalling Antivirus (Sophos).
Changing DNS servers in the client configuration.

Approximately 6 users out of 20 are having this problem.  It is reproducable and consistent with these users.

Answer : Some clients can not access certain websites.

Issue has been resolved.  Problem was with the Telco provided T1 router.  MTU setting was deleted by telco and all sites are now accessible.

Thanks for your suggustions.