Outlook 2003 error "Can't open this item. The item was found to contain a virus and was deleted."

I have a user that is running Outlook 2003 on Windows XP Professional. She has a few emails that are in her Outbox and will not deliver. When you try to open one of the emails, the following error is displayed:
"Can't open this item. The item was found to contain a virus and was deleted. For more information, contact your Exchange Server Administrator."  I manually scanned her computer and it was free of viruses. The client has Trend Micro Office Scan installed. It appears to be an intermittent problem, as she is sending and received other emails. A few of the emails are replies, a few are forwards and a few are new. All addressees are in the GAL on the same Exchange Server. Please let me know if you need anymore information. Thanks!