I've found, and many agree, that Ubuntu is a good distro for beginners. Simple, easy to use and a great community, one of the fastest growing distros and the most searched for on google

. If you want a server computer or anything else apart from a normal desktop OS there's a thread on that in the tech forum.
Linuxes aren't as hard as many people think, there is no need to know any programming language and installation is a snap. The desktop environment is much like Windows (I refuse to un-capitalize)[depends on the DE though), and you'll spend 98% of your time in a GUI. The only time you'll need to bring up a console is if you wish to install something, in which case you'll probably need to type a few make commands or something. It is very easy to learn and Linuxes are very intuitive, so there should be no problem learning how to use it. Google has most the answers anyway.
I like to say that Linux is much like Windows, not security- or stability-wise, but the way it looks. The two desktops are very much alike with Windows being more restricted. Anything deeper than the look of it though is completely different, but many Windows users can be shocked to see how similar the two OS's look.
The links for ubuntu are below, ubuntu has gnome as a DE, kubuntu has KDE. No difference as such, it really doesn't matter which one you use. If someone says one of them is better, laugh at their face cos it's at best only a matter of opinion. There is also xubuntu, in case you have a low-spec computer. That has Xfce4 for a DE, which is a lightweight DE suitable for low specs.
Ubuntu also has a LiveCD for you to try out.
http://www.ubuntu.comhttp://www.kubuntu.comhttp://www.xubuntu.comAs for your original question about Slackware, I dunno

But AFAIK, it is more difficult than Ubuntu.