Well, think about it for a moment.
If an app consistenly quits, there's a problem with what is being loaded.
If it only quits when opening specific documents, then chances are its the documents that are the problem.
If several apps all quit, randomly or upon opening, it's more likely System software, permissions or hardware.
If it is one program that quits, repeatedly, then it is either that program, or the things that program automatically opens on startup (plug-ins, fonts, prefrerences). So the obvious step is to replace the application, the preferences and/or the plugins or fonts.
First: repair permissions. Test.
*Second: drag the preferences out of your Library (~user/Library/Preferences/com.adobe.photoshop.plist and any similar). Test
Third: Remove and reinstall Photoshop from the original CD. Test.
*note: Effectively, this is what logging into the Mac as a different user accomplishes, that is, using a different set of Preference files.
