Q: Powerbook g4 keeps restarting after  logging in

Unhook any peripherals, then reset the PMU and PRAM, and post back if that doesn't fix it, with your OS X version.  Include any changes you've made via software/software downloads/hardware and whether this just started.

PMU: http://support.apple.com/kb/HT1431

PRAM:

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.