Error message with email attachment

We have a user, running WinXP with Office 2007, with a 4 month old problem and I can’t find a fix.

She gets an error message (see below) when attaching a Word or Excel document into an email or when sending an email directly from a Word or Excel document.

I remoted into her computer and saw the documents are on her personal U: drive so only she has access to them. I also saw she didn’t make any changes to any document after attaching it to an email. And, if she clicks yes, the email will send.

How do I make this message stop popping up? Why did it start suddenly 4 months ago?

Here is the error message:
The attachment from the message is open or in use by another application. If you continue, you will lose any changes you made to the attachment that is using the other application. Do you want to continue? Yes No