I asked about the type of security to make sure that you were using something decent. It does not matter much to the connectivity issue since Windows 7 should automatically detect the type of security being used. If there is an option on the router/Wireless Access Point (WAP) to use WPA2, then stick with that. But I would not change anything on the router since your other devices are connecting. One of those other devices should be able to tell you the type of security that it is connected to... I just don't know how to tell you to get that into screen by screen.
Is this a laptop? Can you move the computer so that it is close to the Comcast router/WAP?
I've not dealt with a lot of Comcast users, but I have worked with more than one AT&T users that was attempting to connect to their neighbor's SSID because the SSIDs are somewhat similar. Even if you only have one SSID showing, my thought was that the computer might be too far from your WAP and too close to a neighbor's. In other words, if we are attempting to connect to the wrong WAP - then we will never connect. I know that this is very unlikely, but I figured I would ask. Also, "too far" is unique to the device connecting. e.g. an iPad might connect from a long ways away, but a desktop with built in Wi-Fi might need to be much closer.
So, how many signal strength bars are showing on this problematic computer?
I have 5 out of 5 :-)