Powerbook G-4

I apologize in advance but this will probably be a little long b/c I want to try to relay everything I can about my PowerBook G4 problems so you can figure out what's wrong with it.

I bought this G4 PowerBook used several years ago. It's been a good computer, but I did have to replace the hard drive about a year ago.

A month or so ago it started making a little buzzing sound on startup that would last about 15 seconds then go away. I wasn't having any other problems and it wasn't acting erratic in any way so I didn't worry about it.

Then last night I was online and suddenly my wireless mouse quit working. I replaced the battery, reset the mouse, etc. and it still wouldn't work. I tried to do a force quit of my browser and that didn't work so I had to do a hard shut down. When I turned it back on, I heard the dong sound, but the screen was black. I tried to start up from my Drive Genius II CD, my system CD and I also tried starting up in Safe Mode, but it would never come up. I shut it down for about an hour and tried to start up again and this time it came up normally, but within 2 minutes it was frozen again. So I waited until this morning and started up using my Drive Genius CD and it came right up. I started running the hard drive repair mode and within 2 minutes it was locked up again and wouldn't come back up. The keyboard won't work, the mousepad and the wireless mouse won't work and the screen stays black. However, whenever I restart I always hear the dong and it sounds like the hard drive is doing it's job so I'm thinking it probably isn't a hard drive issue - especially since the drive is less than a year old.

Can you tell by the above information what my problem might be? It is about 6 years old so if it is fixable, do you think it is worth fixing?

Thanks a lot!
Kennie