Phil,
Start with repairing permissions. According to Apple, occasionally the user permissions associated with files or applications become damaged, which can prevent a file or application from working as it should.
See this article. So, you might want to try to run Disk Utility (Applications > Utilities > Disk Utility). Open the application, select the mounted volume for your hard drive and click "repair disk permissions."
If that doesn't help, then you can always try moving the plist file out of the Preferences folder to see if Safari will re-start.