Message: AC Adapter not installed

Hello,

I have Inspiron 1505, I've been using it already a year and I've never had problems with the adapter or the battery. Today after I turned on the computer a message came up saying that the computer cannot recognize the AC adapter and the computer is going to work slow and the battery won't be charge. I run a diagnostic test and everything was fine and then I went to system info and there at the power info there was a message AC adapter not installed. Anyways I got into the operating system and the battery was charging and the computer doesn't seem to work slow. But I get this message every time I restart it. What does it mean? Do I need to get a new adapter or is there something wrong with the laptop.

Thanks for your help!