So, that proprietary 3D graphics driver you installed is now causing you some problems, so you got rid of it. But there's a problem. Your KDE 4 installation had those fancy desktop effects enabled, which now aren't working. Instead of a KDE desktop, you have a nice white screen.
There is an easy keyboard shortcut you can use from within KDE to quickly toggle the desktop effects on and off if you have a problem with them which causes KDE to be unusable.
Simply press Alt-Shift-F12.
Note that this only toggles the setting for that session only. If you want to turn the setting permanently off, you must then go into System Settings > Desktop > Desktop Effects.
I haven't verified this tip in KDE 3, but it may well work. Please do post a comment if you can confirm or deny this.
UPDATE: that would be F12, not F2. Sorry! Thanks to Karper for pointing that out.
