Why won't my computer stay in Sleep mode?

I'm running WinVista Home Premium on an hp m8100n. I have Norton Protection up and running 24/7 (until the free trial runs out!). Windows is set to run updates nightly at 3am (factory setting), and regular maintenance is set at various times throughout the week, all at late night/early morning hours.

I have the power set to go into SLEEP mode after 15 mintues of inactivity, which it does just fine. However, I can't figure out what is causing the system to be ''woken'' from its sleep mode every 10-15 minutes, night and day. When it does this, It also causes the monitor (a 26' flat panel LCD Insignia hdtv) to turn on and eat up even more electricity all night.

HELP ME!! How do figure out what is causing this issue?

I'm worried my electric bill is going to be absurd if I can't get this to stop. (and it's annoying me!!!)

Why won't my computer stay in Sleep mode?moto guzzi

If you would of went with a 28'' monitor you would not be having this problem.

Why won't my computer stay in Sleep mode?norton internet security 2008

You didn't share your new Mt. Dew energy drink with your computer again, did you?
Is anybody calling you? Because when computers are in sleep mode when somebody calls it should wake up.