Why can I no longer open the Security & Privacy pane of System Preferences?
Try applying a combo update

You might try: 


download the most recent MacOSXUpdCombo10.7.3.dmg
log in as zoomq 
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install the combination update
restart the computer, you should find 11D50d or greater 
run Software Update until all other updates are installed
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quit System Preferences
reopen System Preferences, retry the preference pane.


Background

At OS X Lion Update 10.7.3 (Client Combo) there is no hint that Apple published different downloads with identical names. Differences between 11D50b, 11D50c and 11D50d are not published by Apple. This situation surrounding 10.7.3 is most unusual.

Generally, without focus on 10.7.3: in some problem situations, people find that installation or re-installation using a combo has a positive effect. 

In this case: the b in your current installation, 11D50b, probably indicates that a combo (combination) update has been applied once already — maybe through the Software Update routines that are integral to OS X. Reapplying the same update, or a more recent build of the same update, should do no harm. 

There may be more technical ways of investigating causes of your problem. However: because the symptoms are unusual, I am inclined to suggest a combo update before anything else.