Comptuer wont automatically sleep - XP insomnia

So I have my machine set to turn off monitor after 10 minutes (which works) and to sleep after 45 minutes (which does not work). Sleep/standby mode works fine if I manually tell it to - it just doesn't happen automatically.

I'm guessing there is something on my machine preventing sleep mode, but I don't even know where to start. Is there some sort of program that monitors for activity that would keep XP awake?

Do you have a really sensitive optical mouse? Because if that thing moves, just a little bit, it'll reset the timer for Windows to go into sleep mode.

Otherwise, there may be some kind of background process running?

I don't believe it to be the mouse - my screen stays off. I'm thinking windows must have at least two timers for inactivity

the first would be time since last mouse or keyboard activity - this controls the monitor turning off... there must also be something to bypass that programs can use because my monitor doesn't turn off during DVD or TV playback. In any case my monitor turns off and remains off after 10 minutes, touching my mouse or keyboard turns it back on... I don't believe either to be my problem.

I'm guessing another timer must watch CPU or Network, or hard disk activity to guess if the computer is doing something. Something here must be causing my issue, but I can't for the life of me figure out what exactly that is.

Yeah, I have that exact same problem. I just turn off my monitor and leave my PC alone because I don't trust its automatic timers anymore.

Goto your eventlogs in windows, it should report if there is an error stopping it from sleeping. Also goto device manager in windows and look through the devices to see which ones have

"Allow the computer to turn off this device to save power"
"Allow this device to wake the computer"

Typically you want almost all devices like LAN/USB etc etc to be able to turn off to save power and you wouldnt want them to wake the computer either. Typical exceptions to this would be USB remote controller and keyboard