The Problem
For the past several months my computer has been coming out of stand-by periodically at various times during the day and night, and for the first time today, I witnessed it going back into stand-by about 45 min after it had come out.  I rarely turn my computer all the way off, and simply put in stand-by when I'm not using it.  Since my computer is in my bedroom, I've noticed more when it comes on by itself in the middle of the night, but it does happen during the day too and never at the same time.  As far as I can tell, it's not really doing anything when it wakes up.

My System
Dell Dimension C521
Windows XP-SP3
Norton 360/Avast

What I've Tried
1. I've looked at the event viewer after most of the incidents and the first record is always for my network card (Broadcom 440x 10/100 INTEGRATED Controller: Network controller configured for 100Mb full-duplex link.).  The NEXT events are not consistent, and often not there since I usually turn it off right away when this happens.
2. I've checked that my network card cannot wake the computer.
3. I ran Norton anti-virus scans and found no infection.
4. I SUSPECTED that Windows updates may be the problem, so I set my automatic updates to "notify but do not download."
5. I suspected that Norton updates may be the issue so after trying to limit Norton's ability to update, I uninstalled Norton and replaced with Avast.
6. I've never set any programs to run automatically, and none are set to do so as far as I can tell.  This has probably been going on for almost a year now, but as far as I can remember I made no significant changes to my system before it began.

Help!
I've searched the internet for hours looking for a solution, but haven't found anyone else listing a similar problem.  If there is anyone who has dealt with this or knows what's going on, your assistance would be greatly appreciated.  Otherwise, I would at least like some guidance on how I can figure out which program/setting is WAKING my computer.

Thanks!

Is your computer losing time?
No, the clock is always correct.Check you Power Settings in Control Panel, see if anything is set to bring the computer out of stand-by.I opened Power Options in the Control Panel, but did not see anything related to a program's ability to bring my computer out of stand-by.  There were four tabs:
1. Power Schemes.  Set to Minimal Power Management which turns the monitor off Never, the hard disks off after 1 hour, goes to standby after 20 min and hibernates never.
2. Advanced.  Some general options relating to icons, passwords and buttons.
3. Hibernate.  Hibernate is enabled.
4. UPS.  The UPS Service is currently stopped. Are you running Skype or Messenger?...If so, they may be "checking in" and causing you to come out of standby.....Shut them down if you have them running and see if the problem persists.Thanks for the suggestion.  I do run MSN Messenger, and come to think of it, this all may have started around the time I installed Windows Live Messenger.  I will shut this program down for a while and see if the situation improves.Well, it happened again.  I've had MSN Messenger off since Sunday but this morning my computer was on when I got up.  The event log showed the following entries under System:

Information   6:27:58AM   Source: bcm4sbxp   Event 9
Description: Broadcom 440x 10/100 Integrated Controller: Network controller configured for 100Mb full-duplex link.

Information   6:27:58AM   Source: Tcpip   Event 4201
Description: The system detected that network adapter \DEVICE\TCPIP_{84283A25-EE3F-44C0-A8AF-4F2F07910085} was connected to the network, and has initiated normal operation over the network adapter.

Information   6:28:01AM   Source: Browser   Event 8033
Description: The browser has forced an election on network \Device\NetBT_Tcpip_{84283A25-EE3F-44C0-A8AF-4F2F07910085} because a master browser was stopped.

Error   6:28:03AM   Source: W32Time   Event 17
Description: Time Provider NtpClient: An error occurred during DNS lookup of the manually configured peer 'time.windows.com,0x1'. NtpClient will try the DNS lookup again in 15 minutes. The error was: A socket operation was attempted to an unreachable host. (0x80072751)

Error   6:28:03AM   Source: W32Time   Event 29
Description: The time provider NtpClient is configured to acquire time from one or more time sources, however none of the sources are currently accessible.  No attempt to contact a source will be made for 15 minutes. NtpClient has no source of accurate time.

These events were listed in the event log under Application:

Information   6:27:57AM   Source: Dot3Svc   Event 15500
Description: The network adapter has been unplugged.
Network Adapter: Broadcom 440x 10/100 Integrated Controller - Packet Scheduler Miniport
Interface GUID: {84283a25-ee3f-44c0-a8af-4f2f07910085}

Information   6:27:59AM   Source: Dot3Svc   Event 15501
Description: The network adapter has been connected.
Network Adapter: Broadcom 440x 10/100 Integrated Controller - Packet Scheduler Miniport
Interface GUID: {84283a25-ee3f-44c0-a8af-4f2f07910085}

Information   6:28:02AM   Source: Dot3Svc   Event 15503
Description: Wired 802.1X Authentication was started.
Network Adapter: Broadcom 440x 10/100 Integrated Controller - Packet Scheduler Miniport
Interface GUID: {84283a25-ee3f-44c0-a8af-4f2f07910085}
Connection ID: 0x00000001

Information   6:28:17AM   Source: Dot3Svc   Event 15505
Description: Wired 802.1X Authentication succeeded.
Network Adapter: Broadcom 440x 10/100 Integrated Controller - Packet Scheduler Miniport
Interface GUID: {84283a25-ee3f-44c0-a8af-4f2f07910085}
Peer Address: 000000000000
Local Address: 0013723081CB
Connection ID: 0x00000001
Identity: -
User: -
Domain: -
Reason: 458755
Reason Text:
Error Code: 0

The time on my computer was correct when I got on.  Does anyone know if there is a program or something that will tell me exactly what my computer is doing when this happens?  I know there must be a setting in some program that's causing this I just can't figure out which one.Boot into setup...enter the BIOS and disable "Wake on LAN"...I'm not very familiar with BIOS, but I believe this is already set.  The ability to wake was dissables in the power management settings for my network card.