You may have the issue that surfaced recently (December 2010-January 2011) that the standard FAQ http://support.microsoft.com/kb/977116 steps will not fix.
First, you will need to change yours DNS servers to resolve the issue. The theory is that somewhere along the line, caching DNS/proxy servers have corrupt/bad information that impacts some, but not all US users.
Go to http://www.dnsserverlist.org/ which will automatically determine the best DNS servers based on your IP address. Pick two and carefully get the IP addresses.
Next, you will need to change your network settings on your Windows Media Center PC(s) to use these. This will override any settings on your router and/or the DNS settings provided by your ISP via DHCP:
1. Start Menu, Control Panel
2. Select "Network And Internet"
3. Select "Network and Sharing Center"
4. Select "Change adapter settings" on the left side of the screen
5. Right-click your network adapter and then select properties from the menu. (For example: "Wireless Network Connection" or "Local Area Connection")
6. On the Networking tab, double-click "Internet Protocol Version 4 (TCP/IPv4)"
7. Check "Use the following DNS server addresses"
8. Carefully type in the DNS Server IPs you want to use.
9. Click Ok and Ok again to exit.
After doing the above, open an administrative command prompt
1. Select start Menu, then Programs, then Accessories
2. Right click "command prompt" and select Run as Administrator
3. Type into the command window
and then press [enter].
4. then type in:
cd \ [enter]
cd c:\Windows\ehome [enter]
start /wait mcupdate -uf [enter]
Wait a bit and Netflix and Internet TV menus should appear when you open Windows Media Center
If the above does not work, please post to http://thegreenbutton.com/forums/t/97522.aspx?
I suggest you watch this thread/subscribe to it via email in case a permanent solution is found so that you can revert your DNS to normal settings.