Corrupted preference file.
Open a Finder window, type in com.apple.safari in the search field, press return.
Throw the file com.apple.safari.plist into the Trash (it will stay in that window because it shows where the file is) and empty trash. Start up Safari again, reset your preferences.
Your bookmarks will be safe, so no worries there. [img]smile.gif[/img]
Also, this should have gone into Mac Help & Troubleshooting, so you know for next time you have a problem.
(I'm a sticker for keeping things where they should be, but you won't get in trouble if you don't).