Attachements are stored in a temporary path when opened. The one you updated may be in that path but disconnected from the e-mail. The only way I know of to get to that path is to type the full path into windows explorer. You cannot browse to it.
Open Windows explorer and browse to your Documents and Settings\Your User Name\Local Settings.
Then add \Temorary Internet Files\OLK8.
The path is in a registry entry called OutlookSecureTempFolder for Outlook 2003.
The path should be something like this:
C:\Documents and Settings\<your user name>\Local Settings\Temporary Internet Files\Content.Outlook\<some rare name>
You can do this to find out what the temp path is: Attach any text file to an email, open it from the e-mail message and modify it and then close it. Outlook will tell you something like: “The text in <path to the file> has changed…”. From that message you can see where the Outlook temp folder is. Your excel file will be there too.