How does Windows determine system inactivity for suspend?

Archived from groups: comp.sys.laptops (More info?)

I'm trying to have the system automatically suspend after some period of
inactivity. But I can't figure out how Windows 2000 decides if the system is
inactive or not.

I'm VNCing into the system and not using the keyboard or mouse. Sometimes I
can boot up the system, not touch it, not have anything running other than
VNC server (but no clients connected) and it will refuse to automatically go
into suspend.

Other times I will be actively using the system via VNC and it will suddenly
decide to suspend.

I assume Windows looks at keyboard or mouse activity as indication of
activity. But what about CPU utilization, disk or network activity? In
another case I was running Winamp and it decided to automatically suspend.
Does CPU utilization have to be above some threshold to make the system seem
"active"?

Do BIOS power management settings have any effect on Windows power
management?

Thanks