I'm not seeing the Sharing option in System Preferences. Here's a screenshot of my System Preferences:
I'd like to disable the Web sharing option so that I can start Apache.
I'm on an iMac running Mac OS X 10.6.4 (10F569).
A few possible solutions to try:
Install the Mac OS X 10.6.5 Combo Update.
This method is more comprehensive than a simple 10.6.4->10.6.5 update and will more likely fix your issue.