Vista Mail - attachments are completely gone?

I have two machines running Vista in my office. When I send an email with an attachment (pdf for instance) from one which has Microsoft Outlook, to the other which uses Windows Mail, the email arrives with nothing attached.

There isn't even a notification that something has been removed for security. Under the Options/Security, I've unticked the box which "Do not allow attachements so be saved or opened which could potentially be a virus".

I receive attachments from other people to this computer, but not from the computer in my own office.

This used to work fine, it only stopped a week or two ago. I don't think I've downloaded any updates or anything in this time, which could cause it.

Any ideas? We use email to share documents between the computers as this is the fastest way ( we haven't setup a network yet...) and this is getting really frustrating.

Thanks
P