laptop issues (PSU?)

Hello, I have a laptop that I am having problems with. It's an HP and when you press the power button all the lights come on and the fans whir for a moment, but before it goes into post it cycles off. then it will keep trying to turn on over and over doing the same thing that I just said. It does this with or without the battery in and with or without being plugged in when the battery is in. It seems to me that this is the PSU (and hence the system board) but I wanted to see if there was any possibility that I may be missing of some other problem that would cause this. please let me know.