My old Dell computer never does this, it always connects to the Internet on the same network without fail.
It seems to happen more when the computer's been asleep for a longer period of time. I have to disconnect and reconnect to fix it. Wifi card disconnects and shows an error after coming out of sleep: About 1 out of 3 times when the laptop comes out of sleep there's a yellow exclamation sign over the WiFi sign in the systray (bottom right of screen) and the on-mouse-over bubble reads 'No Internet Connection' (but it's still connected to the network).
