What software can prevent a Mac from deep sleep?

The Mac goes to sleep on its own without problem, but unless I select sleep in the Apple menu or press the start button, the fans don't stop.
Galerada wrote on :

I'm having a hard time trying to find what causes my dual processor 1.8 G5 (first series) with 10.3.5 (last security update not installed) to not enter deep sleep. The Mac goes to sleep on its own without problem, but unless I select sleep in the Apple menu or press the start button, the fans don't stop.

It's not hardware related: If I start up from an external FW disk with a plain vanilla 10.3.5, the G5 enters deep sleep automatically. And I've disconnected all the peripherals. So it has to be something installed in the internal disk. And it's system wide, because it affects all the accounts in the machine.

According to your experience, what software prevents a Mac from entering deep sleep mode?

Thanks to all from reading this and for any help you can provide.

G.