I'm another Slackware to Debian convert
I had actually learned about GNU/Linux from teaching myself how to program in Java. On the Java site it told about all the OS's Java runs on. My first distro was ZipSlack, because I'm on dialup and I wanted the most GNU/Linux with the least download time. After testing it out for about a month, I was already starting to brag about how great an OS this was. Soon after that, my dad said something like, "Please don't bother me with this thing. In another month, you'll be bored of it and move on to something else". Well that was the kicker. I was able to ditch MS completely after about a year. (Also, I've quit Java in favor of Python)
As for my dad... I got him dual-booting Ubuntu about a month ago.