Any reason why you chose Slackware? From what I've read (haven't tried it myself) it's one of the harder distros to set up. Lots of compiling. Not "out of the box friendly." Same with Gentoo as well. Out of the 3 distress you mentioned, Fedora is by far the most "out of the box friendly." I have tried it.
If you are just looking for a "different than Windows" experience, any Linux distro will provide that. However, if you're just starting out, I would mirror others' opinions and go with Ubuntu, Fedora, or Mint. Others that might be worth a look are PCLinuxOS and Zorin OS.
Although I currently don't dual-boot, I am running Linux Mint 10 (Julia) on two different machines. Best Linux distro I've used to date. YMMV.