I'd suggest you start with the Fedora Core 1 or 2.
http://fedora.redhat.com/
It's graphical and more like "Windows world". It gives ideas of what is different in Linux than in Windows. it is more soft landing.
Slackware is really good and stable, but it might kill your enthusiasm when it boots to textbased mode.
I myself started with Redhat and journeyed from there to "more advanced" distributions.
Good luck.
