I don't agree that the battery should cause it. The computer should POST with no battery inserted at all as long as it is plugged in. I guess you could try removing the battery itself and plug it in to just see if anything is up with the battery. It could also be the power supply itself. Have you gone to any HP forums to talk about the symptoms with some experts? Also, try
AnandTech: your source for hardware analysis and news. It's a forum with much broader expertise than here.
When you start it up, do you hear the hard drive spinning and everything? Are you sure it couldn't be the monitor?
Unfortunately, one of the things that makes notebooks nice to own is also the reason I don't like them when they don't work and that's all their tiny (and expensive) components.