By design, Outlook will prompt you to archive your old items. You can say Yes or No each time the box comes up, or select an answer and tell it not to prompt you anymore, thereby using that selection for future archiving operations.
Personally, I find it useful to go back to events which happened in the past and so I have always selected the no archive option. Others have archived, only to wish they hadn’t.
Here is how you (the network admin) can set things up so the entire network is affected using Group Policy.
If you have Microsoft Server 2008 or newer, simply follow the instructions here and you will be able to get setup quickly.
If you have Server 2003 like me, you will want to download the files from the link on that projectenvision site and then copy the .adm files (for outlook only use outlk12.adm) to the inf folder in your Windows folder.
Then, when you open up Group Policy Management, you will see the new options listed under User Config–>Admin Templates–>Microsoft Office Outlook
You should know what to do from there, and as always, use gpupdate /force to apply right away, otherwise wait till the user logs off and on and the new GPO is applied.