Windows Update set to update at 3:00 am but doesnt

Windows Update set to update at 3:00 am but doesnt. I have to manually update and it takes forever. I have an AD domain and 100 PC is there something I'm missing. There are all set to auto update at 3:00 AM
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Since you haev so many computers at one location you may want to look into Microsoft Windows Server Update Services (WSUS):

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Can you set one computer to update at a certain time you are there? Just to see if that computer updates?  Also what do the different event logs say around that time?
There are a number of useful ideas here you could try >
Automatic Update for Windows XP:

also ...

Automatic updates don't work:
You may not get your answer from reading this but it may step us in the right direction. In my experience with setting up a task for Auto Update I've had to go several nights of failed tasks before getting it right.
First of all to help define the problem, can you find out what the status and last result of the scheduled task is?
Assuming you are using Windows to do the task, open up Scheduled Tasks in the control panel. If you change the view to "details" you will be able scroll the window and read the status (which may not show anything if the task isn't currently running), and the "Last Result". Last result is a bit cryptic but the information there can help you figure out what went wrong. Of course you may need to search the code or post here for further help.
Also I will tell you I've had issues with tasks not running because of logon and password settings. In the properties of the specific task you are trying to run, check to see If  "Run only if logged on"- is unchecked. If so, you will need to enter a proper usrname and password. I understand you may have already learned this by setting up the task originally. Even if a user is logged in at 3AM it will still require a user and password if 'run only if logged on' is unchecked
I think it's unchecked by default.
Obviously checking the box 'run only if logged on' will let the task run wthout extra credentials. Of course you need to be logged on.
In your case you probably are having users logout at the end of the day.
Also if you don't have a password set, I don't think a blank password will work. A user has to have something in the password field in order for the task to start. At least that's what I've experienced.
Hope this helps a little
If unresolved, scroll to sub-heading "Reset, Repair and Reinstall the Automatic Updates System", there may be something there >

Seems unlikely i know, but have you recently done any repair installation common to all of these PCs ?   If yes, see here>

"windows xp update":  Automatic Updates stops working:
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Windows XP

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