Also (subject to confirmation) if you rename the hidden .kde4 folder in your home directory to say .kde4OLD the system will recreate a new .kde4 on the next login. However, anything in the old .kde4 folder will be ignored, eg kmail messages.
This actually works. Anytime you wipe an application's corresponding hidden folder (or move it/rename it), the system will create a brand new one with default settings. When your user account starts having troubles, and you can confirm it's the user account only, you can use this method to solve just about anything. In fact, if you really bork it, you can go in from root or a LiveCD as root or user using the root service menu in Dolphin and wipe all of your hidden folders and files and completely restore your user account to default settings as if you just installed the OS. Every application you open will create a new hidden folder with default values. Remember to save your email and favorites when dealing with hidden files for your email client and browser. That's where those things reside, as well.
This is one of the beauties of Linux. Windows can't do this trick. I don't know about Mac, but I doubt it does, either. A system that can heal itself to some degree like this is priceless...
If you want to get really extreme (again, from root or LiveCD), you could rename (mv) your entire home folder (user directory located at /home/YOUR_USERNAME/) to, say, /home/old.YOUR_USERNAME.old, and then
chown YOUR_USERNAME:YOUR_USERNAME /home/YOUR_USERNAME
(Obviously (or, maybe not so obvious), replace "YOUR_USERNAME" with your user name )
log back in as yourself (now with a blank slate), and bring files over selectively from your previous, old folder.
Lots of ways to swim a penguin!