Hi and welcome to the forums
Can you give us some more info:
What model PowerBook do you have, and what speed is it?
How much RAM?
What OS version?
What size hard drive and how much free space?
Did the slowdown happen suddenly (like after installing a software update or application) or over time?
Are there any applications that work normally?
Also, take a look at Utilities>Activity Monitor>Show All Processes>Sort by % CPU usage and see if there is a process hogging the CPU.
Good luck