I don't think I even have the CDs for this game anymore, much less the serial key. Is it attached to my account at arena.net like many other games? I'm getting GW2, so I'll be playing that later this week, just thought I would give GW1 one more run through.
So long as you remember your password, username, and a character name, you can play just fine (though, not having the access keys will be a problem if you've forgotten any of that or ever get hacked.) You can download the client from the website: http://guildwars.com/support/downloadclient/default.php