Mac won't stay asleep

My computer won't stay asleep when I put to sleep.
It will sleep for about 10 seconds and then wakes up. The screen will continue to sleep but the hard disk stays awake. I've got the Energy Saving set to 15 minutes in the System preferences. I"ve turned off all automatic updates.

I'm using a G4 tower with 20 inch Apple display and I'm on a cable modem and I use Airport Extreme.
I'm using that latest OS and the programs that I use everyday are MS Word, Mail, I tunes, ICal, Address book, Safari or Firefox. I've got a USB hub that I plug an Ipod into and a digital camera and have tried disconnecting that with no luck.

I can't seem to figure this one out.
Anyone experience something similar?

Thanks for any suggetions.
Ed