I have a GATEWAY NV78 laptop- brand new-purchased late March- it will NOT run with AC power when the battery is attached. It will only use the battery power, until eventually the power runs out and then it will shut down and I can start it again and it will use AC power then - it won't charge the battery though, while it is on. It charges the battery while off. Gateway just sent me a new battery - but it hasn't changed anything. HEEEEEELLLLLPP
It seems to either be an issue with the AC adapter or the power jack. If you just bought it in March, your best bet is to send it into Gateway and have them fix it (it's under warranty yet, so it should be free).