Regarding the first question, my response would have been that it was related to the file associations. The most common cause of this problem is when a user right clicks a file and clicks on "open with" and they choose another program while leaving the check mark enabled for the new default application. This can be repaired by manually updating the registry but its a lot easier to repair within the GUI.
The second could be DNS related but could be a multitude of issues. Using NSLOOKUP is the correct tool to use to figure out if the correct IP address is being resolved for the host name. Generally when some websites are working while others are not is not a DNS server issue. It could be virus related where the local HOSTS file has been modified or proxy related, website being down, or user error.
JorgeM
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