G4 Won't Boot

So, I'm the owner of a Powerbook G4 - 1.25 Ghz, 15 incher, 512 Mb ram, a 160 Gb HDD, and superdrive. The problem I am having is that it won't start OSX. It was up to date as of two weeks ago. I installed MS Office '08, and after the next sleep, it sketched out, and I tried to get it to sleep again, and when it did, the sleep light was about twice as bright as normal (though that could be me seeing things). Anyway, I rebooted it, and it gets to the gray Apple logo, WITHOUT the spinny thing.. And it just sits there. This notebook did NOT come with any disks, so I can't reinstall. I've tried zapping PRAM, and that stopped the freezing, but it won't boot. I can get to the boot device menu, I can get it into target disk mode, and access open firmware. Also, verbose mode says nothing. Normally, were this a Windows laptop, I'd have had this solved within the day.. But I'm still new to Macs. I _do_ have an older iMac G3 if it can be used to revive the Powerbook. I've tried yanking out the PB's HDD and repairing, and it won't work. Any help would be appreciated.