I now have a frozen G4 after turning the computer off during an installation

While trying to download a book off a website, I clicked the wrong download button, which turned out to be an advertisement and started installing an unknown program instead. I didn't know how to stop the installation so I turned off the computer. That was dumb, I know. But now when I turn the computer on it is frozen with the apple on the screen and the loading circle just continuously spinning. Can someone talk me through unfreezing my computer so I can remove this suspicous downloaded program from my computer. I want to save my old Powerbook G4, from 2004, it's been good to me, and I'm not ready for her to go out like this. I can't tell you more specs because I don't know them off the top of my head, and the computer is frozen right now. Thanks so much.