Outlook Can't Open this item

A group of three users in a lawyers office send tasks to each other through pop mail accounts. It has been working great for over a year. Now when one user sends a task to the two others it gives this error when trying to open it.

Can't open this item. Outlook cannot do this action on this type of attachment.

I did some googling and found that the tasks must be sent in Outlook rich text format. I went into Contacts (like directions said) and changed the format setting for each user to "Send Using Outlook Rich Text", but the problem persists. They are all running Outlook 2003 sp2 on newish p4 systems.

Has anyone seen this before?

I fiigured it out. It was AVG anti-virus. I reinstalled and changed the email plugin to Outlook plugin rather than Personal Email plugin

thank you for including the answer, but was this the purchased version of AVG or the free version?

Paid Version