K, so I install KDE and set it as the default GUI. I restart. I log-in and am presented with...

a completely glitched out, scrambled interface. Huh? Any guesses as to the problem? I'll post a screenshot as soon as I can (performing tasks on a computer with a mutilated interface can be difficult.)

Sounds like video card problems, to me. What card is installed? And could there be any settings from an older install interfering? Give us as much info as you can on what video hardware you have and anything you can think of that could be relevant.

You can also try some experiments like interrupting the bootup and adding parameters - I think "video=vesa" would give you an ugly but working desktop. If it does, then we are definitely looking at adjusting your video. Also, at login time, if you get that far, you will probably have an option to run KDE in Failsafe mode - everything unnecessary turned off. That's worth a try. Once we know whether any of this helps we may be able to get down to specific help.

What distribution and KDE version are you using?