Software

If they are doind cache, you could disable "perform an automatic send/recieve when exiting".

I've had this happen on a couple XPpro machines as well but not for very long. The Outlook icon stays in the sys tray for half a minute or so.

If this happens again, look to make sure that only one instance of OL is running in task mgr.

Do these group of users have larger than normal mailboxes? PST's not corrupt?

I'm assuming some connection with the server is causing lag since the app itself should be fine.

I agree with jguasch in that it might be trying to send/recv before exiting, check that out by pressing Ctrl+Alt+S to bring up the send/recv settings box... if the issue is still happening after that, you might want to try to do a detect and repair. I had the same problem for one of my clients and after that it seemed to work just fine. To do a detect and repair go to Help > Detect & Repair.

let me know what happens