For a lot of email clients, it's as simple as double clicking on the attachment
icon.
If you use Outlook Express, Outlook or Netscape Messenger (the one's I've used),
there should be an icon either in the header or at the bottom of the email body.
And if you have the program on your pc that's associated with that file's
extension (.xls = Excel, .doc = Word, etc), then just double clicking on that
icon should open that program and open the attachment.
Every once in awhile, the email program (or windows??) gets confused and forgets
what program owns what extension.
But you can usually right click on the attachment and select Save As (or some
variant), then save it to your local harddrive (make a note where you saved it)
and finally open your program and do File|Open to open that file.