Most Linux distributions has already contained KDE. But KDE may not be the default Desktop Environment. If you use distributions such as Mandriva , OpenSUSE , Kubuntu etc. , you may have alerady installed KDE by default. But if you are using distributions like Ubuntu , fedora , debian etc , you may be using GNOME because GNOME is the default Desktop Environment of these distributions. However , no matter which distribution you are using now , you can install KDE Software on you own.
If you are just considering switching to Linux...
for the most end users
If you are just considering switching to Linux platform , you'd better choose a distribution contains KDE as the default Desktop Environment.If you want a system easy to get started , distributions like Mandriva Linux,OpenSUSE etc is quite great. These distrobutions are easy to install , you can just install them without removing your Windows system. After a try you may considering replacing MicroSoft Windows with Linux platform and KDE Software. These distrobutions use KDE as their default Desktop , they also contains many other software such as openoffice.org , Adobe flashplayer and some non-free hardware drivers. These packages give you a better experience of the operating system.
But if you want nothing but pure KDE Software , you could try Chakra GNU/Linux, but please noticed that Chakra may not be suitable for most end users! On the one hand , Some packages useful for end users such as flashplayer is not contained in this OS , becasue Chakra is gtk2-free. On the other hand , Chakra haven't provided the function of automatic partitioning , this means you must partition the disk manually. It's a bit difficult for new users.
for intermediate users
If you have been using Linux...
If you have been using Linux , you may be using GNOME , XFCE or LXDE. Just have a try on KDE Software! It's quite easy to install KDE Software on most distributions.Also, you can also use KDE Software with other desktop environments such as GNOME.
If you are still use MS windows or Mac OS
For MS Windows or Mac OS users , maybe intall a Mandriva Linux or OpenSUSE is a better choice. You can install them without removing you MS Windows Operating System. However, KDE Software is also available on Windows now. Have a look at KDE on Windows project.