What you need is a WSUS[Windows Server Update Services] setup inside your LAN.
The WSUS server will automatically download updates from windows update & the clients in your LAN will get the updates from the WSUS server.
If i'm not wrong then you need Windows 2000 with sp4 or Windows 2003 Server to install WSUS upon it.
Download WSUS from here
Best of all its completely FREE !
To know more about the WSUS head over right here: WSUS FAQ
I'm not acquainted with it yet,cause i havent installed it.Will do it by today night & get back to you.
But other than that there is simply no chance of getting updates to all the clients available.
Oh btw you man try the GFI Languard Scanner,if i'm not wrong then it does a fair enough job of scanning & updating computers through windows update.
Though this may not be what you want in the first place.
My suggestion would to read up a bit & get WSUS working.
GET SUS installed on server and then change auto update path of the client machines using group policies to local server path..and then enable auto update