Close System Preferences, then look for a file called "com.apple.preferencepanes.cache"...it's in <your user directory>/Library/Caches. Drag it to your Desktop and then try opening System Preferences again. If all looks well, you can go ahead and trash the file you dragged to your Desktop.
This problem seems to pop up every once in a while on these boards. I believe the solution is either to reboot or trash a preferences files but someone else will have to fill in the details.
This problem seems to be cropping up on the Intel systems fairly frequently...not sure why. I don't think I've ever heard of it happening on a PPC system.