When a colleague tries to open attachments in her email (Outlook 2003 talking to an Exchange 2007 server) they talk ages to open. The files are relatively small, all less than 1MB.

We've tried creating a new Windows profile for the user and tried creating new Outlook profiles, however that hasn't made any difference.

And we've tried accessing her account from someone else's PC, and the attachments open immediately there.

The only thing that might provide a clue is that Process Monitor shows Outlook on her PC trying to write the file to a folder within the user's "Temporary Internet Files" folder with FAST I/O DISALLOWED errors. Can't find a lot of useful information on that message online though.

What causes the FAST I/O DISALLOWED errors?

And would that make opening attachments so incredibly slow that opening a < 1MB file can take a matter of minutes?

UPDATE: Discovered that this isn't just an issue with Outlook. Other files being accessed over the network show the same FAST I/O DISALLOWED errors in Process Monitor. The problem is just more noticeable with Outlook, because although other applications take a while to open files it isn't a matter of minutes.