Thank you all for your support.
As I mentioned earlier in my topic I do not like to mess with partitions during the install. The chance to make mistakes is almost 100% for me. I do not have experience with partitioning. That is wy I choose to do it with g-parted or diskdrake (thanks Textar) and delete the Mint installation completely so only "unallocated" remains. After that the install is a piece of cake is my experience.
And now how I did it: Started the Live CD and started as Root. Opened Synaptic, pressed "reload" and installed G-Parted. The rest was a piece of cake for me.
Now PClinuxOS 2011.6 KDE is running fine and fast (!) in a dualboot on my MediaPC.
Now Add Locale, Libre Office, updates etc.