Raid had a good thought, but at least on 10.5 clicking "Show All" will also clear the search field, so that's probably not what's going on.
The first thing I'd try is repairing permissions; if somehow the OS doesn't have permissions to read the pref panes and/or their enclosing folder, that might cause this. If you've never run Repair Permissions before there are descriptions everywhere (but basically open Disk Utility, select your system disk, and click the appropriately-labeled button).
Next thing to do would be to go to /System/Library/PreferencePanes (note that this is the Library in the system folder at the top level of your hard drive, not the Library folder in your user folder or the Library folder at the top level, either). It should have about 30 items in it for all the stock preference panes; if it doesn't, then some rather vital system files somehow got deleted and it's probably time for an Archive and Install to repair whatever else might have happened, though I'd try to identify the cause first.
If everything there is in order, and a Repair Permissions and restart don't help, do you have a separate user account on the machine you could log into to see if it's ok? Normally I'd suggest creating one, but of course you can't get to the relevant preference pane...
