Programs only unexpectedly quit if either their preference file is corrupt or if there is a directory or system level issue with your computer. The former is fairly easy to diagnose:
1. go to Home -> Library -> Preferences and locate the files with plist in their filename's suffix that refer to the application that quit and move them out onto the desktop. Remember some files may have the developer's name and not the software title name itself. In addition, some people have found that deleting the com.apple.quicktime preferences after the recent Quicktime updates has fixed the problem. Note to Quicktime Pro users, a side effect of deleting the preference may be the necessity of re entering your Quicktime Pro code.
2. Try reopening the application. If you get to at least the registration code asking screen or the program launches, you have fixed the problem, though may have to reenter the registration code.
3. You may also want to use Apple menu -> System Preferences -> Accounts to create a new administrative user and login under them. If you login as the new user and the application no longer unexpectedly quits, then you have missed deleting a preference which caused it to unexpectedly quit in your original user login under Home -> Library -> Preferences. If the application unexpectedly quits under the new user, then you have a system level issue which can be a preference in:
Hard drive -> Library -> Preferences
which could then need removal. Other system level issues are discussed below:
Non-preference issues are a little more complicated:
1. Boot off the Mac OS X installer CD or DVD that came with your computer.
2. Go to installer menu -> Disk Utility -> First Aid tab
3. Select hard disk, and select Repair Disk (note: this function only repairs minor directory issues).
If errors occur, hit repair disk again up to 4 times until all errors are gone.
4. Quit from the Installer menu, and restart the computer.
5. If errors didn't occur at all, and you still have problems, you may have to clear the system cache with the freeware:
and run the cron scripts using it as well and update prebinding for the system, as well as repair permissions.
6. If errors were not able to be eradicated completely, restore your system from the last backup you made. For those who have no backup, I strongly recommend getting a Firewire or SCSI hard disk depending on which built-in port you have on your machine and backing up with either http://www.dantz.com/products/mac_express/
and verifying the backup is bootable. Do this frequently, and do it before you run into a system problem that compromises the directory completely. You may be able repair disk directories with http://www.alsoft.com/ 's Disk Warrior if the Disk Utility can't, but don't count on it as a substitute for having a backup handy. Disk Warrior can repair the directory of systems whose directory only has been munged. Any more extensive damage will require recovering from a backup.
In the worst case scenarios you may experience kernel panics. Follow those instructions to solve them.