Replacing default WordPress login method

I have a site with a working login system already in place. I used this as the base and have been heavily modifying it, http://www.evolt.org/node/60384. I was wondering if there was an easy way to replace the default login setup on WordPress to my custom one. Could I create a plugin that could achieve this? If so how would I? Or would I have to modify the core files? If so, which files/ functions? I am new to WordPress, but I know php, html5, Javascript, JQuery, and more. I am willing to learn!

Thanks,
Legobear154

Also, if this is the wrong forum I am sorry, nust tell me.