Maybe an obvious hint, but nevertheless worth the try if you notice that a) you have random kernel panics, b) applications crash for no apparent reason and/or c) apps won't launch, especially as one user and not if you log in as another user.
Assuming it's your own Mac, log in as root, create a new user account with administrative access. Now delete the 'unstable' user. Mac OS X will ask you who will have access permissions for the files of the deleted user. Assign these to the new user.
If you want the same login name as the deleted user, launch NetInfo, choose the user section and adapt the 'new' user settings by changing all the login names to the old name. Also, rename the new user folder to match the new login name.
Log out and back in with your new account and start copying your stuff from the 'old user DELETED' folder in the Users folder to your new user account. Of course, grab everything from Documents (you might wanna hold on to that Microsoft User Data). You can then start copying Preferences (from the ~/Library folder). Be sure that they are ok; corrupt profiles are cause number one for troubles.
If all is done, delete the 'OLD user DELETED' folder. You should now have a 'fresh and minty ' user environment, and this in less than 30 minutes.
[Editor's note and caution: This hint is an extension of a tip regarding creating a new user for troubleshooting purposes. Be careful if you elect to replace your admin user; the process is a bit more complex, and maintaining the same username does require some work with NetInfo Manager. I would recommend this as a last resort if you've tried other fixes without any luck. Of course, if this step fails and you still have system problems, then you've either copied over the item that was causing the problem in the first place, or you've got a much deeper hardware or software problem.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020529093352807