I hate to break it to you, but your iMac probably runs an OS that is entirely different than what would ship on a new Powerbook. We're not talking the same type of difference between say Windows XP and Windows 95, but Windows XP and Windows 3.1. That iMac probably has OS 8 or OS 9 on it. There have been 4 major Mac OS releases since OS 9 - 10.0, 10.1, 10.2 and 10.3. Don't be fooled by the point releases - each point release has been a major upgrade unlike in the Windows world where a service pack (the equivalent of a point release) is not considered a very big upgrade. The newest OS, OS X 10.3 would run on your iMac with 128 MB RAM but it would be very slow. Interestingly enough, 10.3 is faster than 10.2 which is faster than 10.1 which is faster than 10.0. So if you try any version of OS X on this machine, stick with 10.3.
In regards to your original question though, try searching Apple's site for instructions on how to reset your iMac's "PMU". That's the power management unit and often is the cause of start-up troubles.