These instructions should be good for web-email on a Windows98, Windows 2000, or Windows XP machine.
The first item to accomplish here is to get the file onto your computer.
Open your email message as you normally would. You should be able to see a file attachment in the email - most likely located close to the subject line (although in some email programs the attachments are located below the body text). It may be different in your system - look around.