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If it is the DNS, try clearing the Resolver Cache. Go to Start, then Run. Type in the dialog box cmd and press enter. When the command line screen appears type in ipconfig /FlushDNS, and press enter. Exit and try the siytes again.
If acklan's suggestion doesn't work it's possible that your friend's ISP's DNS servers aren't working. These are the services provided by your internet company that translate website names into actual addresses. If they're not working, you can use a different DNS service. My ISP doesn't like to assign the DNS server addresses to my DSL modem, so I just use the servers over at OpenDNS since they tend to be more reliable anyway. Here are the instructions on how to set it up: http://www.opendns.com/start/windows.php