There’s a good reason Akismet is bundled with WordPress: without it, we’d all be drowning in comment spam. It’s impressively effective: there’s not much spam that makes it through to the comment moderation queue, and it rarely marks genuine comments as spam. (You should give your spam list a quick once-over before deleting it, though, just to make sure.)
The only trick to Akismet is that you need a WordPress.com API Key. To get one, go over to WordPress.com and sign up for an account. You can have an account at WordPress.com without getting a blog there: check the “just a username, please” box. Your confirmation e-mail will contain your API key. Save that in your password program, and you can use it in Akismet for every WordPress installation you set up. (Don’t use your own key for clients, though; set them up with their own accounts.)
Once you’ve done that, moderating comments will be a much easier job.