If you are having trouble opening some web sites, the problem may be your Maximum transmission unit (MTU)
An MTU is the maximum packet size in bytes sent across a network. There are multiple factors involved with determining the MTU, I won't get into that in this article at this time. However, if your network has a mismatched MTU you may have a problem opening certain web sites or even all web sites. The MTU is configured on the Network interface itself, in the registry. You can use regedit the edit the MTU, or you can download and use DrTCP (Zipped format). Calculating the MTU, at this time the maximum packet size on the Internet is 1500 bytes minus 28 bytes overhead for setting up the packet. So if your network is set to an MTU of 1500, the maximum ICMP packet will be 1500-28=1472 bytes.

Testing your MTU from the problem workstation start by pinging the Gateway of your internet router from the workstation.

ping -f <router gateway IP> -l 1472

You will get one of three responses; the ping will return, you will receive "Packet needs to be fragmented but DF set." or it will timeout.
If you the ping gets a timeout, that means a downstream router has a mismatched MTU, and is the probable reason for your connectivity issue. Incrementally reduce the 1472 until the ping returns.
If you get the packet needs to be fragmented but DF set, at a low number of less than 1400, see if you can increase the MTU without a timeout. Sometimes you have to just settle for a happy medium, but ideally you would really like a number as close to 1500 as you can get. I've seen an MTU as low as 528 bytes on a PPPoE connection, if you shared this connection across a network you would need to set the MTU to a much too low of a number and it would affect your network performance.

How to change the PPPoE MTU size in Windows XP

Connectivity Problems on ICS Clients When You Use a PPPoE Connection on a Windows XP ICS Host:

Windows CE FIX You cannot download a file over a PPPoE connection to a server that requires encryption: