Outlook Express and opening an xls file

I have a client that has outlook express. One of his co-workers sends him xls files to his mail. He cannot open the file in outlook express. It says that it is not associated with a program. He can open all other attachments. When he copies the attachment to his desktop he can open it with no problem. Do you know what could cause the behavior. He should be able to open it in outlook express. I did remove the security feature(opening files that could be considered a virus).I thought that is what it was but no.
thank you for your help