Many times people will leave a company, and the request will come in to "delete their account". I usually recommend NOT.
It's important that due process is observed here. First, the account can be "Disabled" while the user is leaving the building, or being escorted out, or what have you, but it should be re-enabled, and the password changed. This will allow people to continue to email this user without getting bounce-backs, and will afford someone on your staff the opportunity to sift through the emails to make sure there isn't unfinished business. Also, be sure to log into that user's computer AS them, to make sure there are no files in the "My Documents" folder, or on the desktop that you need.
The user's mailbox should remain, but further mail should be forwarded to someone who is taking over the old user's work load, or a supervisor to make sure communication doesn't break down between your company and your clients, or vendors, or whomever.
Once you're certain all business that user was handling has been completely transferred to someone else, you should archive the user's email and files to DVD. Make three copies, one that stays accessible, one that goes into a firebox, and one that goes offsite.
Doing this will save you lots of time (and thus money) down the road. Restoring files from a backup should be a last resort. Wouldn't you rather have the files you need available, rather than trust in your backup in case you need them?