Concerning your connection issues,

What operating system are you using? I've had similar problems with XP home in the past (installed from a XP restore CD).

If not start with the easiest thing first

Go to http://www.puffinsoft.com and try running the DXport program before starting mercs.

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If you still have issues try opening ports.

-Here are the ports that are required to be open:

For MW4 clients, UDP port 28800, TCP/UDP ports 47624 and 2300-2400 should be open when the game is running (Multiplayer/Connect).

For MW4 servers, ports 9999, 27999, 28805, 28806, 28807, 28808, 47624 (TCP) and 2300-2400 (TCP/UDP), 28800 (UDP).

How to Open Ports in the Windows XP Internet Connection Firewall
http://support.microsoft.com/default.aspx?scid=kb;en-us;308127

(copy & paste)

Best of luck to you

_____
Remember not to take off your blinders until the ride is over.

Free Cuba!