Sounds like a DNS problem to me as well. What you can try to do, is to enter command prompt (type cmd in run) and type the following:
ipconfig /flushdns
It will clear the local DNS list, it happens sometimes you get invalid references making trouble there. If that doesn't work, consider contacting your ISP as they may have trouble with their DNS.
You can also try openDNS, if this works it's most likely the DNS servers of your ISP having issues.
http://www.opendns.com/