Sounds like a job for Apache's mod_rewrite. In your .htaccess file, it would be something like (I say "something" because I've not tested it and don't claim to be an Apache guru):
RewriteEngine On
RewriteRule ^[a-zA-Z0-9_\-]+$ index.php?id=$0
Then in index.php you would use the value in $_GET['id'] to determine the user and generate the user-specific data from the DB accordingly.
Try some googling on things like "apache mod_rewrite tutorial" for lots and lots of info.