Outlook dials ISP even tho set to LAN

I recently setup a mail server on a network and removed the external ISP mail accounts from some PCs. Mail accounts were set-up to connect using Local Area Network in Outlook Express 5.5. All worked fine except one PC insists on dialing the ISP usedfor connecting to the internet. I have reinstalled everything and yet it still dials. An exact duplicate model of this PC works normally. I have tried everything I can think of. Any ideas anyone?