By "quitting out" do you mean you're getting the "Program XYZ quit unexpectedly" message? If that's the case, here are some things to try:
Are only certain programs affected, or all programs? If only certain programs do this, try moving their preferences and other data files to a folder on your Desktop (or anywhere other than where they're currently stored - and by data files I mean stuff in the Library folder in your user folder associated with the programs in question) and try again.
Create a new, testing user account in System Preferences, log into that one, and see if the problems still occur. If they don't now, the cause is in your user folder somewhere.
If those two steps fail, try repairing permissions. Open Disk Utility, select your booted disk, and click Repair Permissions.
If that doesn't work, boot from the Mac OS X install DVD and verify the disk. If errors are found, have it repair them. Keep doing this until you either get "The disk appears to be OK" or a message similar to "The underlying process reported failure on exit".
If you got the second message (or something like it), your disk has errors Disk Utility cannot fix. I hope you don't get here... because, if you do, it's time to either turn to a more advanced disk repair utility or format the disk and restore from a backup.
