Outlook 2003 and attachments

a few of our users are having an issue when they try and open email attachments that have been sent to them, they receive an error message stating
"Unable to open one or more attachments"

If they click this 5-6 times it will eventually open the file.

This doesn't relate to any specific file types as it happens on all of them

We run office 2003 on Windows 2000 professional and Microsoft Exchange Server 5.5

Just wondering if anyone else has experienced this problem or knows of possible routes to a solution, as im now at a loose end