Problems with connecting to every network

So I've had this computer several mknths, and this issue only started flaring up a month or so ago. I click on the wireless connection bars, which opens up the blue list with every available connection. However when I click on one (or any) of these connections and click connect, nothing happens. The dialog with the checkbox asking whether I want to connect automatically just disappears but the networks window remains open. When I go somewhere where it has a known network that I've saved that it recognises, it automatically connects (this is at two places: my house and my parents'.)
I used to just ignore this and not see it as a problem because I rarely carry my laptop away from home. It's proven to be quite frustrating now though as I've just replaced my modem and it refuses to connect yet any other computer in the house connects fine. Is there any way out of this? Could I add my new network to the known network list without the blue dialog?

Have you tried forgetting the networks then adding them again? You can add networks manually using Network and Sharing in Control Panel.