You have just updated a desktop with an intel graphics card to KDE 4.5 and find that Desktop effects no longer work or cause your system to freeze after a while.
After the update, you may have had a 'solid' panel instead of a transparent one.
First, undo any changes you may have done while trying to solve it.
Now go to:
Configure your desktop -> workspace appearence and behavior -> Desktop effects
Check "Enable Desktop Effects" if it isn't already.
Click the "All Effects" tab.
Uncheck the checkbox next to "Blur".
If all goes well then Desktop effects should be working now.
Current open-source drivers for intel graphics cards do not support the opengl effect needed to create the Blur.
Compiz users already know about this issue because their own "Blur" plugin is almost useless.