You need EXCEL installed or the excel viewer installed.
Excel does not come with WinXP it is in Microsoft Office or you can buy it separate.
If the attachment is greyed out (no access), go to TOOLS -> Options ->
Find where the line is
Do not allow attachments to be saved or opened...
and uncheck it.
I have installed Window XP on my computer but when I click the attachment with Excel file in Outlook Express, it doesn't open. I have seen people using the menu ( on the top of the window ) to adjust it so that they can see the file. But I don't remember exactly where it is. Can anyone help?